It is worthwhile to note: while I know of games with bigger casts of characters* (Dota 2 has 121 characters, LoL 155) they've been built over the years through the live service model - it took both those games ca. 10 years to reach this point. I can only see Smash reaching that number of characters if it adopts a live service model - and I doubt Sakurai would be keen on the idea. He's not exactly IceFrog, a guy who embraced the game he develops to the point he's been willing to commit 15+ years of work into the game. Also, there's the Bandai-Namco staff to consider in this case since Nintendo contracts them to work on the game, so... yeah. It's difficult to do that with any other dev studio as well.
*(Intentionally discounting cases like Tobal 2's 200 characters since that game has a lot of pallette swaps etc.)
... **** it, turn Smash into a live service game just so that this thread will be eternal.
Edit: Aaaand suddenly rumors are swerving again surrounding WB Games:
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