So is Body Damage how much damage you take or how much damage you do to their body? If it's the second, what's the difference between Damage and Body Damage? Also the speed is a fun trick, the issue is that they get inside you and you can't hit them. Maybe put some kind of collider on your body so they can't enter?
Oh bosses would be so cool! As far as making levels different, would making levels that are mazes be too much? So here's my experience: when I enter arenas that are harder, I stick out the initial wave at the entrance so if things get bad, I can bail. Then I gotta do a nail-biting run across the field since I don't want to be stuck. Making it not as simple as "Run to the other side" would be cool. Doesn't have to be a maze, but maybe sudden sections of walls? Random large groups of dudes rushing you with perhaps a warning? Maybe secret side rooms?
Maybe some newer enemies like ones that run up, unload a lot of firepower, then run away, perhaps dudes that orbit you while firing, dudes that buff other dudes, maybe ones that when they die, they lave behind a fire crater that damages you, maybe dudes that slow you or do damage over time? And maybe instead of circles, they could be like triangles to show they're different? I don't think I've gotten past arena 30 yet.
Do you have any plans for new weapons/tank bodies? Would be cool to have like "Apocalypse: +20% damage and range, -30% move speed" or something to suit your playstyle?
God this game is shockingly addictive!