Good gameplay, various enemies, graphics is great, smooth death animation, but you should add pathfinding to enemies and stop them from shooting in walls.
P.S. also add power ups.
Healing objects from destroyed tanks, damage upgrade, speed boost, more loot(for future). Making power ups or upgrades can be done very differently: you can make temporal buffs, leveling up, upgrade items. I want to say, you need to think what progression system will be(or won't be) in the game and after that what it will upgrade. But that's only my opinion and I'm not very experienced developer.