Honestly, I feel like an underrated pick for a "single-franchise platform fighter" is Minecraft. First of all, you have Steve's building mechanics that can really shine with a full platform fighter built around them. Second, the sheer variety of items in Minecraft means that you can pretty much have a whole roster full of Steves while still being pretty varied. Sure it would be a little lame if it was just Steve variants, but it has pretty much the biggest roster variety when just "variants of the main character." Luckily, there's still tons of potential with the other mobs in the game. The Enderman and Iron Golem are probably the most obvious picks, but there are tons of other mobs that could work such as the Wandering Trader, Illagers, Spiders and Spider Jockeys, Guardians, Drowned, Snow Golem, Blaze, and Slime. And that's just the first game. There's tons of other mobs from Dungeons, Earth and more. Sure it might be a little weird to see these mobs placing and destroying blocks, but the concept of a Minecraft platform fighter in general is kinda weird tbh. Nonetheless, I still think it would be incredibly cool.