My suggestions:
- Focus on polishing core game loop aspects, balance and feel, it needs to be challenging and engaging.
- Develop consistent art style and change setting, maybe experiment with 3D art (you can check out Unity's Tower Defense Template).
- Add VFX that makes it easy to distinguish which towers hit enemies.
- Add enemy healthbars.
- Add tower stats info when clicking on tower, show tower range, also info about upgrade cost and changes.
- Add abilities that go on cooldown (ex: drop a meteor in clicked area).
- Add risk-reward mechanics like: investing gold in "mines" that give you more over time, summoning wave ahead of time to get more gold.
- Make less cells for towers to balance the player's power spike.
- Make levels, start game with small and simple ones, introduce new enemies and mechanics gradually.
- Star-scoring at the end of level based on how much HP you've got left.
- Add bosses.
Basically - look at all the TD games you like and analyse them thoroughly, see which common things you must have, which unique mechanics you like and what you may experiment with. And don't be ashamed of copying - everything was created before, we remix things, we put old stuff into a new context, thus making it unique and with our own flavor!