Turret ideas
freeze turret, flame turret, slam turret to deal area dmg, stun turret.
Monster ideas
Stun monster to stun turrets. Initial 5-sec cooldown upgrade for more coins. initially worth 30 coins.
Boss Monster for a lot of money, but takes 15-sec spawn time. 2000 hp, and can also stun. stun cooldown 15 secs. initial speed is 0.50. Initially worth 5000 coins.
Bolt Monster. extremely fast but has low hp. initially worth 15 coins.
Healer monster heals other monsters in a radius by initially 25% of hp with 10 sec cooldown. for boss, heals 5 hp. upgradable. Initially worth 50 coins.
Rebirth ideas
You should add a part on the turret rebirth upgrades that you can upgrade their base stats in. Also, you should add upgrades that let us exceed the cap value.