Team Chat without being grouped.
As you know, riot has put into effect the ''Lobby voice chat''. They should honestly just make it for everyone in your team, at least for Solo/Duo ranked games. Communication is very important in a game and from 8 years of experience in this game, you lose a lot more games because no one knows what the other players want to do.
Seeing as you would still be able to mute the toxic players and turn up or down players with louder or quieter mics, it shouldn't be a problem. It wouldn't improve toxicity, if anything it would decrease it. Most players can't talk in a mic due to their anti social tendency's anyhow, thus the whole ''keyboard warrior'' phase. It also prevents players from accidentally flashing or healing while typing/ stopping in lane to tell players what is going to happen. You can just tell everyone while continuing to farm or fight what's going to happen next. Even if players don't want to use the voice chat to talk to their team, they can still listen in with the headphones I assume everyone uses while playing your game.
THIS is probably one of the most important issues in league right now. I get there are bugs you need to be working on, and champion balancing. But this needs to happen first. It would save a lot of players from losing free ladder points because of lack of communication.