some suggestions for a more completed version:
work on the economy (a single coin per round isn't viable)
two modes infinite (like this appears to be) and a story mode with finite waves and an alternate ending if victorious (maybe have multiple opponents/characters for different difficulties. Multiple opponent extreme difficulty modes (dvp and tvp versions for more targets.)
better explanation of the towers of which there is basically none in this version.
updating the audio is a given
add more tower types (sniper, artillery/aoe, slow/stun, for example)
replay/new game button at the end of loss scene.
Edit: a couple more ideas/details
next wave display (just cosmetic but would be a pair of balls filling up before wave and with a cell count next to it.
sense the current opponent(Tyrone) seems to be a general athletic type the others could be a surfer (hard difficulty) and some variant of jock (moderate difficulty) making Tyrone the introductory opponent.
(If it already existed I missed it) in-game pause function.
possible modifiers? examples: modifier:virility double opponent sperm count but half their health, modifier:taketwo doubles the wave amount and the players money (gained and starter.) modifier:hotboost opponent has a chance to proc a boosted wave (extra enemies with basic health) bonus points if a visual indicator is given to boosted wave (increase staging zone cum and more visible in said next wave display.
modifiers would be per game and not wave based. (either toggled or pure chance)