technically speaking we are already there
Fortnite:
Xbox One - gen8
Xbox Series X - gen9
Switch 2 - gen10
but I'd also bet once the next gen Xbox is out, you can add it to this list to have 3 xbox generations in one game.
PS6 will apparently release later, so maybe by then the Xbox One and PS4 versions will be discontinued... but honestly, there's no technical reason they couldn't be supported for a long time, given the Switch 2's similar power profile and that Mobile Fortnite is still a thing and supports far lower specs than the PS4 and Xbox One.