Personal Champion Pool Bans in ARAM
As someone who almost exclusively plays ARAM now, one feature that I feel Riot could implement without having to worry about reducing matchup diversity would be introducing bans to your own champion pool. Yes there are definitely some characters who excel in this game mode by virtue of their kits, but I would argue that if I am able to guarantee that I don't get a specific set of characters that I loathe playing then my overall experience will be consistently better regardless of what other people end up with.
Personally, I feel much more strongly about the character I get than the ones my teammates or enemies get rolled into, so I feel asking "which characters don't I want to play" would be a much more appealing way of helping to improve this mode over limiting the champion pool for everyone.
I don't know how feasible this approach would be, but it's something that I would definitely encourage the development team to at least consider.