Some things I want to add to the discussion.
I also wanted to have Ava / Coop for more than four people. I think it's doable, but the balancing might be a problem. If there are that many people sending creeps, single target towers may become impracticle. You could increase their stats, but that would make them to strong in 1on1 where atm. splash damage already isn't that strong.
So it needs a fine balance or maybe as I said different tower factions. Which would work until some players get eliminated.
I could see people arguing, that it takes skill to read the board and play the right towers, which is true to some extend, but I am not sure if that's the whole picture here. Still... I also want 6 player games.

MJochen also mentioned mobile devices. I am also split about this.
I think it's the best way to attract casual players and it might be fun. It's the future somehow...
But it also limits what we can get on a desktop version.
Dcode posted a mobile-video on discord. You can see the game working there. Nice.
But you also only see one map. I don't know how good you could see the whole game; the maps of the others and play accordingly. If you want to see everything, you might have problems with placing your own turrents / change their settings.
But that aside, there might be other constraints a mobile version would bring to desktop. Atm. I am not creative and can't name an example, but I am sure there will be stuff. I don't think the game can get to complex / to timing based with an UI that has to fit on a mobile screen. So we need to be careful there.