So, concept: Super Smash Bros. RPG in the style of Super Mario RPG. It's got the same ring-shaped-world-map-with-conspicuously-themed-regions deal, except each region is based on a different Nintendo property: there's the Mushroom Kingdom (of course), Kong Island, a miniature Hyrule, an orbital colony where Samus Aran hangs out (probably a late-game region, that one!), and so forth.
Now, here's the question: owing to space limitations and the need to stay on brand, the game is naturally going to have a major focus on first-party Nintendo characters and properties, but you get to include exactly one world map region based on a third-party property, the only requirement being that at least one of that property's characters must have appeared as a full-featured fighter (i.e., mirror characters are okay, but Mii Fighter costumes don't qualify – sorry, Sans!) in a Super Smash Bros. title.
For reasons what would obviously take far too long to explain here, you get to make this decision. Who do you pick, and what does their region of the world map look like?
(Note: no chaining multiple third-party franchise crossovers together. i.e., you can pick Sora, but that doesn't also get you Mickey Mouse.)
25 years ago from today, Super Smash Bros. was originally released on the Nintendo 64 in JP, making the franchise reach its 25th anniversary. It was developed by HAL Laboratory and published by Nintendo.