In response to your concerns, I believe that arena will become rather popular if they merely implement a decent AI and also allow people to fully customize their characters. But to address your suggestions,
#1 is an awesome suggestion. +1
#2 is really interesting, and could be pretty cool.
#3 sounds like it would work really well. Another awesome suggestion.
#4, how are you going to implement this? What rewards? I don't think this would work very well, since there'd be nothing to reward that wouldn't affect performance, besides maybe a little cosmetic badge like Evolve has.
#5 is amazing. Yes. All my yes. This would be so huge.
And as for balance, I believe it is supposed to be highly lethal, and this is early access anyways, so I wouldn't worry about it too much. Once we're able to customize our characters, we should be seeing more survivable units, and they'll probably nerf weapon damages anyways. Hopefully they nerf armor too, and leave us with long matches that are played out with limited resources like armor kept in mind.