You know how dedicated MMO players say that the game is “just beginning” when you reach the level cap? Hyrule Warriors is a bit like that in some respects; completing the main story mode is far from the end of the game.
In fact, one might argue that the “endgame” of Hyrule Warriors, if you choose to look at it in that way, is in fact the most substantial meat of the package. Eschewing the narrative focus of the main story mode in favour of a non-linear, mechanics-centric experience that is all about just having fun as you see fit, this part of the game will keep you busy for a long, long time.
And, moreover, it will do this not by forcing you to grind the same things over and over again, but instead by offering you a huge variety of different, highly replayable challenges that each put their own fascinating twists on the core mechanics of the game.