Idea is good and game is fun (finally, a TD that is letting us maze again instead of the same old just putting turrets down). One thing, it's more like an idle TD when you come to the point your map is fully build, the only thing I have to do is click the upgrade after every round (perhaps having the option putting this on auto too would help). Also, can't seem to exit my run, so now just waiting until I fail, but the monsters have never even reached the first corner, so I'm afraid this'll drag on for a while. Nonetheless and again, good starting point for the game and see a ton of potential!
Some ideas that you likely already thought of:
1. Track the upgrades you've selected and see the list of active buffers?
2. Make % upgrades more rare?
3. Make it possible to reinvest the tiles (become obsolete after fully build maze), e.g. by being able to add rows/columns by selling tiles or something?
4. Perhaps making it possible to combine towers / hybrid tower approach?
5. Air waves so that the path is not relevant for those rounds?
6. Boss rounds (also in infinite mode), e.g. every 10th round a boss that drops a special randomised upgrade?
7. Have a goal for the gold towards endgame?