Another few suggestions, two pills:
Health Up, adding max health to a tower, and Balls of Steel, adding temporary health to a tower instead that you can't recover once lost, like Soul Hearts work in Isaac.
A baby:
Robo-Baby, a familiar that'd shoot a low-damage laser relatively fast that can pierce through an entire lane. Could make for decent chip damage while working with a different damaging tower, would definitely work well with Ghost Baby.