exactly. there are a lot of ways to incorporate this. in fact, it would be simpler as just a level and not a game mode, but I feel like if it was a level, it would be too frustrating :)
I like the Idea of a game mode, that way you don't have to do in it in either story or Endless. Because it's a game mode, I feel that instead of changing maps and enemies, it would keep the map and swap enemies.