A common idea when I'm idly daydreaming about making my own mod is a fully-featured Battle Frontier with modern mons/moves/abilities/etc.. Just drop the story and exploration entirely and have a battling sandbox with a bunch wildly different formats that reward interesting use of the entire series' options. Doesn't even need genius AI, if I wanted my gimmick teams to be crushed by optimal setups I'd just play OMs on showdown (well, aside from the general lack of natdex OMs to play in).
I think the formula used for Ash's goal in Journeys has a lot of potential to be used as a game - travelling through portions of regions trying to climb up through a ranking.
Anyway, I have two things to say.