I think more elites should be added for a larger variety. Some way of mildly increasing damage for
weapons because of the way enemies health scales.
more barrels, maybe a lighting pole set of legs for the turret.( it would drop lighting poles that would zap enemies that walk through it)
A little pop up that shows up when you get a new personnel best.
An upgrade that could increase move speed
Any kind of weapon that has a hilarious amount of knock back and damages enemies when they hit either a wall or each other.