(warning: this is super SUPER long)
review: First thing i want to say this is in my opinion a perfect game. with everything from the mechanic of the game to the towers, the enemies ,and more.
but also are you still working on the game? (not saying you have to.) because if you are ,can you make the lab upgrades? (i am not trying to sound rude).
And also you have time, I have a lot of ideas you could try, but if you don't have time or just don't accept them that is okay as i shall respect your decision and not be entitled .also i have a lot of ideas and would take long to make which explains why i shall accept you option.
but i suppose i could give you them either way just because i really appreciate the game and i feel would be able to make the game (in my opinion "better") but i don't know busy you are, and i don't know how people would react to these ideas being implemented to this game and if it would in reality make it worse
idea 1: make new towers (example: a shield tower[protects regular towers and has high health],or [melee tower]combats enemies in front of them, [bomb tower] deals very high damage but has a very slow attack speed)
idea 2: you could make new enemies(examples :[ranged enemy] hurts towers from a distance. [exploding enemy] :explods itself when it attacks a tower destroying itself and destroying the tower it was attacking no matter how much health it has)
idea 3:make new upgrades that the lab can do (examples: make queue time shorter, make tower attack speed faster, learn new tower, learn enemy weakness.
idea 4, 5,& 6: 4. you could make it themed (examples : medieval, modern, future, etc) 5. or even manage to make a story with it. (don't have examples sorry.) 6. make certain enemies have a weakness to a certain tower (tbh i dont have examples you could do.)
And you could do even more but if i listed all of my ideas this would go on forever so this is it bye.
(also i just realized how long i managed to make this. I can barely do a single paragraph of writing at school before im out of ideas ,i am shocked about this.)