Infinity Blade has this thing wherein they introduce new challenges every week. However, they put certain conditions on these challenges so that you can’t just spam the same strategy over and over again.
I think it’d be cool if RR2 did this with dungeons. Every week, there might be a new Challenge dungeon, and with different requirements. For example, “Don’t use Cannons” or “Beat the base without using Spells” or even “Beat the base using only [3 specific units].” I feel like it would test players to see if they really understand all of the game mechanics, and thus, would provide variety in the game. The rewards wouldn’t even have to be that much; just enough to make the dungeon worthwhile.