So, a lot of franchises that were added in Smash 4 onwards only have one stage. The older franchises have a variety of stages but it always struck me as strange that some of these prominent franchises only have one. Even something as limited as Ice Climber had two stages.
So let's try and make a new stage for the Smash 4 onwards franchise that only have one.
Those franchises would be...
Mega Man
Wii Fit
Punch Out!!
Duck Hunt
Street Fighter
Bayonetta
Splatoon
Castlevania
Persona
Dragon Quest
Banjo Kazooie
Fatal Fury / King of Fighters
ARMS
MineCraft
Tekken
Kingdom Hearts
Mega Man - It's the cliche answer but if we can use content from the other series, Central Highway from X feels like a no brainer.
Wii Fit - This one's a little more abstract but I like the idea of making the Balance Board a stage and taking the Balance part quite literally. Have the stage tilt and shift around during the fight so players have to actively try to keep their "balance".
Punch Out - Either Doc's gym or a scrolling stage based on Mac's roadwork training. For the latter, they could expand upon it and add more locations in the background for other characters to cameo in. You could pass a cafe where Glass Joe is relaxing and drinking coffee for example, or have some of the other boxers also running to train or just walking on their commute doing other things. The key characters would of course be Doc Louis and Little Mac who the scrolling would be following. If Mac is in a match, you'd just follow Doc on his bike.
Duck Hunt - Since Hogan's Alley is part of their moveset, maybe a stage based on that?
Street Fighter - To many good options but I'll go with a Ken stage since he's playable now. That said I would skip SF2 in favor of repping a different game. In this case, I would prefer his Alpha 2 stage celebrating Eliza's birthday with all the other Capcom character cameos left intact. And maybe as an extra easter egg, the system could read your birthdate and change the banner to your profile username on your birthday.
Bayonetta - It's actually been quite some time since I last played the games so I don't really remember the location names. However, I think I'd skip 3 entirely in favor of a stage based on the Cereza game. I adore the art style of that game and it'd make for a beautiful stage.
Sploon - I still haven't played 2 or 3 but I'm sure there's no shortage of good options from both the multiplayer maps or single player campaigns.
Castlevania - I've played very little Castlevania so I can't really give a proper location here either. I do think either a clock tower area or some place outdoors would be nice though.
Persona - If we can pull from any game then I'd actually like them to go back and pull from Persona 1 or 2. Nothing particular in mind here otherwise. If it has to be from 5, then just convert one of the palaces into a stage. Again, no preference.
DQ/BK - Skipping because I haven't played their games.
Fatal Fury - Geese stage or Mt. Rushmore Terry stage
ARMS - Min Min's stage if only because I don't like that they gave her Spring Stadium
MC/Tekken - Don't know enough so skip once again.
KH - The city section of The World That Never Was.