Remove the Slot Machine
I know that TFT is Riot's own spin (no pun intended) on the Autochess craze but the slot machine is easily my least favorite mechanic of this new sub-genre.
And yes, it is a virtual slot machine. 5 champion slots, 2g to reroll, 3 of a kind upgrades, etc... I'm sure it has hooked many gambling addicts. Wouldn't it be nice if Riot changed the slot machine into something more original? Much like the carousel and item crafting, Riot already added some fun twists to the game and I'd like to see a unique twist on the way you acquire champions.
The problem with the slot machine is obvious: it's too random. It may feel good to get that champion that upgrades to a 3 star but you know, deep down inside, that the slot machine may be screwing over another player at the same time with no upgrade options. Is it really fair to give one player 3 Karthus cards before other players even see a single 5-coin champion? Or give one player all the Dravens? Or one player a Cho'Gath at level 4? No, of course not, but It happens. The winner of the top three can be easily decided by who the slot machine favors more.
As to what to replace it with, I don't know. More carousels? A skillshot game? Anything that doesn't feel as random as the slot machine does would be worth looking at IMO.