As in the title, he asked me to have bars over the heads of HP and a little more character.

I'm sorry my English is poor.

Health bar above enemy heads?

I would like this too, but then again that kinda makes the rogue's skill useless (the one where he can see close to death people). The more characters would be nice but it would take a lot of work and balancing (more so then now).

allies - may be
enemies - not gonna happen

I think it's okay the way it is now. Maybe some healthbars with avatar at the side like in a WoW group. So you can fast click on the Avatar who's health is low and see them highlighted on the battlefield and heal them.

Impossible! But the sidebar for the leader, healer or streamer might be a great idea.