Huge wall of text incoming. I tried to organize it as much as possible but.. well... Here goes...
So... I just met another Yi in a ranked game who's obviously abusing guinsoo's so after spending the past 3 months or so trying to understand where Riot is headed I'm starting to give up all hope about this game. Now naturally I'm not saying Yi has a problem. It's everything else.
So, first you give scuttlers 5 stacks for junglers to see if that will create more fights (to make the game look more fun to people watching). You also add another miniboss that gives a buff so that people will fight over it (to make the game look more fun...). You also start creating champions with borderline OP skills that look extraordinary but who lack synergy with most current meta team setups and have next to no real value when it comes to objective control (see jhin - again, cause 1.5k crits look cool - which I'm fine with but the champ lacks compared to others).
You increase several items' costs and giving some extra changes around the place to make up for the increased cost. But you don't increase gold gained from kills. The big - huge problem with that is that you're actually destroying comparably slow moving mages. You're practically removing them from the game. Because they can't really roam safely since vision is much more intense nowadays, they can't easily decide to leave the lane because they will probably lose around 2 waves of creeps (aproxx one kill worth of gold) and kills don't actually get you any more gold than farming so... Mages have to constantly farm. Which they can't because.. well.. Thunderlord's exists... But i'll get into that later...
Itemization. I'll mention every single item I think has issues, hopefully this will be heard.
*Sheen (and all its products): The fact that Sheen's AP part has been removed seems to point in a direction that helps people who don't scale with AP. However, there's tons of champions who had been using it as a core item to help them poke, increase burst or even help them push lanes a bit faster. Now, since it has no AP it actually sometimes sets people back when they buy it, because the enemy would spend almost the same amount of gold to get a hard AP or AD item and give them a clear advantage. Also, there are cases (like Ezreal or Jax) who liked having trinity (and they still can) but the AP part of it is gone, meaning they somewhat lack in what it used to give them. I do however like the cdr part in it so I find it fair to keep that. Trinity is the most expensive item in the game anyway so it seems fair to have pretty much a bit of everything.
*Tiamat: I think that champions who used tiamat really did need it for the clear speed or splitpush possibilities. Removing the passive aoe dmg really makes it fall behind.
*Titanic Hydra: Deals way too much damage on activation. I feel like it should be reduced by around 20% or so to make it feel like it's adding up but it's not actually decimating people. There are countless times when tanks just come in and with the extra damage from this item simply bring a squishy champion to 20% hp from full in just one burst (hell, they can even kill them sometimes).
*Blade of the Ruined King: Supposedly a %hp shredding item. But the damage doesn't feel like it increases enough in most cases when you're damaging a tank. Being a targetted purchase I feel like it should have a larger impact on high hp tanks. In addition to that, most tanks nowadays can sustain endlessly when at low hp, which is when botrk is the weakest, making adcs unable to finish off tanks that simply stand on their head. A slight buff on its minimum %damage might be useful.
*Maw of Malmortius: While the whole idea behind "get spell shield - start berserking" is very very interesting, the fact that it can be paired with a couple other items that provide a similar effect is starting to make this a bit of a must for many champions. And not just Olaf who you obviously seem to consider strong. A slight nerf (5% on each stat) on the buff it gives should bring it to a more normal performance scale.
*Liandry's Torment: again, low damage when target is at low current hp, meaning it doesn't assist in bringing down high hp tanks. Now I get why AP champions shouldn't be able to do that with ease, but it might need just a tiny bit of a buff. Maybe increase the % multiplier by a few per cents...
*Lich Bane: I'd say reduce the base attack damage scaling and increase the AP scaling. That way it's not too good for Kassadin and a bit better for mages who like to use auto attacks (lady of luminosity anyone?). Again, tiny changes.
*Frozen mallet: Switch pickaxe to short sword and ruby crystal to giant's belt. Reduce total AD and increase HP by the amounts relative to the belt and sword difference. That way it still feels like a worthy buy for a tank who likes to stick onto people (MUNDO!) and doesn't set their tankiness back by purchasing a comparably high ad item.
*Banshee's Veil: Relate its cooldown on how much hp the champion wearing it has. The less hp, the lower the cooldown. Not too hard to code and would work like a charm.
*Dead Man's Plate: No wonder almost every tank uses this item. It makes them a threat. Reduce it's 100 stack damage from 200% to 150%. Everything else seems in place.
*Banner of Command: 100% base hp regen and 60 AP don't seem to really work together very well, since magelike supports usually have low hp regen base values compared to tankier ones and tankier ones don't usually need so much AP to scale well. Play with the numbers a bit, or make the item a bit more fun to use, so that we get to see it more.
*Wit's End: Reduce base magic resist by 10, increase maximum stacks by 2. I think that'll make it work more as intended and actually show the user its effect. The damage dealt doesn't seem like it would be too much, considering it requires extra auto attacks which means more time in perilous danger for the usually squishy champions who may use it (teemo?)
Finally reached it...
*Guinsoo's rageblade: Not much to be said about this item. Its pure stupidity is quite obvious, considering people are banning champions who abuse it well almost every single game, and when they don't they have more chances of losing. (Have you checked up the stats for champs like kog? sudden huge rise?) The damage is absurd. Now the passive is fine. But Guinsoo's rage is just way too overpowered. It should basically have the base damage removed, work only with % values, and have the aoe effect get reduced with distance from the target that has been hit. Like the cleave passive on ravenous hydra.
*Guardian Angel: Commonly the goto item for marksmen through the ages, it used to work amazingly well when teamfights didn't happen every two minutes in the late game. Right now, with so many objectives around the map, and homeguard for free, people get back in fight too soon for the item to recharge its cooldown. And while that HAS to happen for it to not be overpowered, it feels like you are unable to postpone a fight because the enemy team can practically just take down your entire base until it goes off cd. A 20 seconds reduction should be enough to keep it on par with other defensive options.
*Zz' Rot Portal: Now I know this one may be a tough one, but if there's any way to make the little thingies not deny farm from team mates that'd be awesome. I get how they should push and all, but if they don't actually last hit, they often make minions die before someone can react, practically removing gold from the map if you get unlucky. A tiny improvement to their targetting script could go a long way.
*Athene's unholy grail: Often you need to buy this item to survive your lane as a mid mage. But when you do buy it as a first item, you start feeling a huge lack of damage. Now surely that's meant to happen, but don't you think it happens a bit more than intended? +10 AP should make it fine.
*Iceborn Gauntlet: Being an item that's supposed to "add a little bit of peel" and a little bit of "threat potential" for champions, it seems to be dealing too much damage when tanky champions with high base ad use it. I think it should be brought down to 110% or so.
*Sterak's Cage: Too strong, Too cheap, Works too well with other items like maw of malmortius. Not many items seem to fit for the recipe and the combine cost is already considerably high but it just has to cost more on total. It's right now too easy to just decide to buy it if you're doing well and snowballing, and then practically be almost invincible. It just costs too little. Another 200-300g on its total cost for such a strong passive should be considered.
*Void staff: Not void enough. 40% would make it a worthier buy when the enemy team is stacking magic resist. Especially since legion aura is practically a must in almost all team setups. You could increase its cost by around 200g to compensate.
*Ohmwrecker: make the active last for 5 seconds and it should become more efficient.
*Will of the Ancients: This one doesn't really have a problem I'd just like to state that I liked the teamplay potential on its old AOE passive, perhaps you could revisit it. Team game after all...
*Frost Queen's Claim: The slow is sort of... Get slowed... Perish in agony... I think it just slows too much... Other than that it's a fun item and a good strategic opportunity.
I frankly think that only thunderlord's has an actual issue at the moment... Compare it to deathfire touch and just laugh at how stronger it is. Not only does it have a huge base damage, it even scales well enough to justify abusing it on every single role. Yes, every role. Tanks use it, Assassins use it, Most mages use it, Supports use it, heck, even marksmen for the early game poke potential or to get an extra kill early on. The cooldown is fine, since it's based on level and all it feels right. But when tanks come at you with: Dead man's plate, titanic hydra, and thunderlord's, those 3 non-champion-specific things can give up to what, 700-800 damage total? Now, when a marksman has around let's say 2k hp late game should a tank be able to deal almost half their health in damage by just stacking flat damage values? It's a TANK... It's supposed to TAKE damage and deal decent damage OVER TIME. Not in bursts!!! even if it's one burst! it's just too much! Frankly adding up the very realistic value of aproximately another 700-800 damage from a full combo that most late game tanks can do right now, it almost kills a marksman or a mage. And yes they should position better, but they should have at least some outplay potential. Making it easier for other team members to pick up a kill after you deal some damage and cc them squishies is just fine, but killing them pretty much on your own clearly isn't.
So. Thunderlord's, from 10 per level, reduce it to 5 per level. And increase its AP scaling by 5%. Ad assassins are fine as it is.
Since i mentioned how underpowered it is compared to thunderlord's I should mention that I think deathfire touch is a fine mastery but perhaps it could get a 5% buff on ap scaling too. Most Ap champions who use it are mages and well mages usually deal aoe damage which means it won't last long. So the scaling on them isn't that amazing. And sure it does get better over time but health regeneration is massive right now so a small dot like this one wouldn't make a big difference if it's increased by a 5% ap scale, it'd just make it more valuable compared to thunderlord's that just deletes people.
Matchmaking: The current matchmaking system on solo queue is a disgrace. Silver playing with platinum is simply not fun at all. For neither side.
Finally I think the current system isn't good enough when it comes to players' attitudes. Sure, people who swear sometimes get chat bans n' all but hey, what happened to positive reinforcement? People who are friendly are helpful need to get some sort of reward for being such people. That's a better way to promote a good attitude. I don't think there's enough of that. Sometimes you are friendly to people or try to help them and if you make one mistake there's always someone who insists that "you shouldn't talk so much because you're a bad player". Which kind of is enough to make you not try to be friendly for a while. So we need something in the actual game to promote good behavior.
I really love the mechanics of this game and how well it plays. It's clean, looks fine and doesn't really bug out that much.
But it also feels like Riot's been trying to make this game look good when you're not actually playing it by adding stuff that synergize well enough together to create the so called "WOW" moment. While that's good to help the publicity of the game, I think more attention should be given when such actions start deteriorating the quality of the gameplay and the level of equality between several paths that players prefer.
I hope this gets the necessary attention, as I spent a lot of time writing it.