Disclaimer/Tl;dr: I don't want to take it completely off the table, I just want it so that they can't freely ban the champion you want to play just because they want to troll.
You know, how after you get your role you select a champion that lets your teammates know you want to play them.
I've had it happen a few times where a teammate just straight up bans the champion I want to play, and the champion isn't considered flavor of the month or even above average.
Now, I don't totally like the idea of keeping it off limits, but if you want to ban someone that your teammate wishes to p lay, could you add a confirmation option? Like how you can trade if you own the same champions, just a little yes/no box that appears near your name if you rally wants to? You never know; maybe you DO want to play a flavor of the month champion, but your last pick. There is an argument to be made for banning him even if your teammate wants to play that champion, they are quite likely to pick it first.
All I want is for trolls to stop banning the champion I want to play for no other reason than they fell like trolling.