I know, I know. This seems like a very surface level question to ask that could be applied to literally every game franchise under the sun. However, as the game continues to swallow up other gaming franchises from Sony and Microsoft, Nintendo is the last holdout. Nintendo is famously very stubborn about sharing characters outside of their games. At the same time, we've seen them in recent years open up with games like Mario+Rabbids (they gave Mario a damn gun!) and Cadence of Hyrule.
For once, I would probably like to see Nintendo be stubborn only because Fortnite is a game I don't care about and is becoming increasingly diluted the more franchises they add.