Although kind of the opposite of building onto a defense post, I can't help but think of an official variation on the board game Battleship when posed with the question of earlier and earlier tower defense games:
Each player starts with five ships placed where one wants, as usual.
However, each player in turn calls five shots instead of one -- as long as that player still has five ships.
EDIT: Iirc, the targeted player then declares how many hits and misses (but not which ones were hits vs. misses).
Lose a ship to enemy fire? All right, now you're down to four ships able to fire four shots on your turn.
You'd better be skilled at tracking down the enemy, or else you won't have much of a fleet left to defend itself.
Fun stuff. $:^ ]
Meanwhile, upon the OP posing the question/challenge of inventing a whole new genre, my mind went into that automatic "answer the question with something" mode, and I came up with (what I believe to be) a pretty cool concept I've never seen before. Sure, it springboards off some other things that have already been done, but whatever: "The thing that has been, it is that which shall be; and that which is done is that which shall be done; and there is no new thing under the sun." After all, where do you think turn-based -- and even real-time -- strategy top-down games would be without chess, or even checkers? If you go read the historical accounts of the influence "Space Invaders"' carried in its heyday, you can check off practically every descriptor for Call of Duty as you go. Etc. As for my concept I've never seen before, maybe I'll get to work on it someday, hmmmm. $:^ J
Regards,
Bob