What I would like changed...
1) More upgrades for tower.
2) Many another tower or two. Would love a splash tower with like poison.
3) Centralized upgrading system:
-- Clicking on the building your defending, like "Improve all ground tower damage by X%" Several 'Ranks' and such. More money or some other system...
-- Maybe as your building get damaged maybe a "Damage Bonus to all towers by X%" as like a frenzy, last dieing effort to kill the mobs.
4) Possible of making unlimited Rank upgrades to the towers.
-- $BaseCost = 100; $BaseScale = 1.75;
-- Every upgrade from the first. Scales up. Since there is never ending enemies, and limited upgrades... It can make the maps tedious, and boring since its gets really really hard to kill things after like Wave 130.
5) Improvement tower
-- Like a single tower has an "Aura" with it, that all towers, or only X amount, get a damage bonus, firing speed bonus, etc...
-- Certain towers give each other tower a small bonus when "Connected" to each other.
6) Slowing effects.
I'll add more later as I think of them.