Well in these games, for me - it is important to have nice stash of different enemies, and introduce them one wave at the time. This way, players will know that there will be something new each wave, and it will keep them engaged. Which brings us to the problem most devs are suffering - the content :-) Speaking of content - think about different worlds, too. I guess you have implemented the world with Unity terrain, and it is really not necessary to go with that. Instead, go to asset store, and purchase some cheap scene assets, then assemble a couple of different scenes, and bake the lighting. It will look pretty decent.
And the last feature I feel is interesting - some powerups spawned somewhere on the map, which will be available for limited time (30 sec for example). Then, you can put an indicator on some kind of minimap/radar, telling players they have some special pickup available. This can put them under the strain and trigger their FOMO and desire for discovering.
Anyway - long story short - just work on the content you feel is reasonable to add, then when you have enough for decent demo (like - 3 different worlds, 5-6 different powerups spawned around), and 10+ different enemies, go and polish your movement mechanics, and you are good to go ;-)
Good luck!