The Battle Frontier in Pokemon Emerald was created as the ultimate set of endgame content to the game, at least in terms of battles. In creating a multitude of alternative battle experiences (whether it be randomized rental Pokemon in the Battle Factory, the cryptic rooms in the Battle Pike, etc.), the game was able to challenge the players for theoretically hundreds of hours, without having to create any new Pokemon.
Always thought it would be cool to riff on the pokemon core idea. Dragon warrior monsters and Ni-No-Kuni are the only games that come to mind are nice examples of that kind of thing. Hmm, maybe there are others I’m not thinking of?