With the increasing popularity of "I'm unstoppable after level 16" I think the following additions would fit nicely with the current game and power creep:
Renekton: At level 16, now generates fury constantly instead of losing it out of combat. In ult form, every ability is enhanced.
Riven: At level 16, become permanently ulted and you can cast Blade of The Exile every 30 seconds (reduced by cooldown reduction)
Karthus: At level 16, your ultimate does true damage
Soraka: At level 16, gain bonus health regeneration equal to 100 (plus 100 per 200 ap) every 5 seconds.
Tryndamere: At level 16, you become permanently ulted. Each time you receive lethal damage, you are stunned for 0.5 seconds.
Ashe: At level 16, your slow becomes a root.
Neeko: At level 16, your ultimate is split into two casts: The first one is a 800 range dash doing nothing, the second cast is the same as the current ultimate.
Nasus: At level 16, Wither hurts the target champion, dealing 20% of max HP every second for 15 seconds. In ult form, your Wither gains 1500 bonus range.
Kindred: At level 16, your ultimate only affects your allies.
Mundo: At level 16, each time you land a cleaver, you're ulted free of the HP cost.
Xerath: At level 16, gain 42 additional ult charges.
Udyr: At level 16, gain a permanent spell-shield and Turtle Stance now has a unique cooldown of 2 seconds (reduced by Cooldown Reduction). Lose all mana costs.
Heimerdinger: At level 16, your turrets become regular Summoner's Rift Towers.
Morgana: At level 16, your Q triples in size and doubles in speed, and roots for 7 seconds (6 second cooldown, reduced by cooldown reduction).