A lot of roguelike bullet heaven games like this have sort of drone/constructor builds and/or weapons, like the weirdo has. I think that would be fun to add. Then the enemies could have something else to target other than you.
Maybe they could fight and swarm enemies. They could either bash into/shoot enemies and do a lot of damage, or be a distraction, moving quickly around the enemies to take pressure off of you.
You could go a different
direction and they could be a support, an alternative to gems getting sucked towards you. They could go out and grab gems and bring them towards either you or the tower.
So many different ways to take them, that's why I love them.