At the time I was thinking something more singleplayer, but having a verses, survival (each has their own “map”), or a director vs player/s (think one person spawns specific type of enemy) would be fun. You could force a grid like system which prevents players from completely blocking a path and enemies from getting stuck. You could use next bot ai or A* for a path finder.
The survival type gamemode I am thinking would be like starcraft 2’s where each player has a map and they fight their own waves to see who lasts the longest.
This is something similar to what I’m thinking.
You could keep players interested by linking their profile to a “player” which can level up towers, maybe start from a previous wave, bonus tower/upgrade unlocks. A leader board would be fun too. I really like the idea of having a first person tower defense in that you do a small amount of damage but you can aid your towers with different strategies, maybe instead of doing damage you have a slow weapon to increase the duration the towers can hit the enemies. You could even do something awesome like pre-generated maps like Melonbomber does, with maps made out of props.
If you do consider doing something like this, I would be more than glad to help with the project, coding entities, helping with anything not GUI or HUD related. I personally don’t want to be fully responsible for a project that gets dumped because I am the only one working on it, I get busy or I simply don’t have the knowledge.