I'll stick with the theme, it's not a creative theme but I think it'll push the dev to their limits and that's what developing is.

So, if it's going to be a tower defense, then my concept is something like this. You as a player invaded by monster or enemies that want your unique blood because it can be used for building technologies like towers and walls and traps and so on. yet your blood is your life, and because there are invaders and you are a lonely single fighter guy, then the only option is to defend yourself by using your own blood!

Your objective is to keep alive! that's it, but how you could keeping yourself alive while you defend it by using your own life? well, currency management is your one way answer. there are shops and building store on the gui that you can buy and build your strategic deployment. it's a lane based tower defense so it needs to calculate where and when your guards should be deployed.

on the enemy side, they'll come steadily until a certain wave then they'll come in a group of badasses.

It's what I got on my mind, not that good but I think it's better than nothing.