Firstly, the biggest issue I have right now is deciding whether to play rift for tokens or TFT for absolutely nothing. It's a great game but this should not have been overlooked, regardless of it being a beta, the amount of time that we want to put into a game for no reward when I pay for a pass and really want the Caitlyn skin.
Okay, from here on, serious game feedback.
The AI is poor. There are instances where my Ahri will kill a target, then throw her spell at the dead target, but the delay is significant enough that I feel the AI should know better than to throw the spell in that direction. There are many instances where characters will not fight because they feel like they cannot reach the target. In most instances there are paths available, but the character appears too lazy to walk that path. Assassins are wildly unpredictable, and very rarely do they target the same champion. I understand they "target the furthest champion" but is not always the case, I believe space has an impact on this and for me, assassins should focus the SAME champion to really assassinate somebody. Right now, players can just put their tanks at the back and be fine with you splitting focus, but even tanks would face assassination vs an assassin team should they focus correctly.
Some luck-based aspects could be minimized. The carousel round for example, could be a shared shop where characters do not need to move to choose the champ/item. This would still allow those who are at the bottom of the standings to choose first, but with no luck about where the champions are positioned when you are finally able to select what you want. It is fun, but as you have mentioned you have plans for a ranked queue to be available, I feel luck should be minimized where possible (within reason).
This is just my small input after a few games, hopefully you all have some other feedback to add.
As for assassins, though I would like them to group their focus, I do feel like they may be TOO overtuned for that, and in turn need nerfing to balance it out, but it is a frustration I find myself having.