So sorry I haven't posted here in a while.
However, there are some things in the actual game that I feel people here might use as headcanon fuel, so I'm gonna share them:
-Master Hand's trophy description states that he "personifies creation in the Smash Bros. universe". Confirming that Smash Bros is set in it's own little world, and Master Hand is it's creator.
-Wario's trophy description contains the following:
"If someone who hated Mario tried to draw an ugly version of him, it'd probably end up looking just like Wario.
The greedy villain claims they used to be childhood friends, though...".
I apologise if that's old news, but I have never heard any description of Wario say this. They usually say Wario hates Mario.
-Some fighter trophies reference the real world:
![Pac-Man :4pacman: :4pacman:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
"The yellow, circular hero of the game PAC-MAN, which is recognised as the "Most Successful Coin-Operated Arcade Game" by Guinness World Records."
![Sonic :4sonic: :4sonic:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
"A certain hedgehog from a certain series you just might have heard of."
![Megaman :4megaman: :4megaman:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
"In 1989, Capcom's new action hero, Mega Man, arrived in Europe!"
![Duck Hunt :4duckhunt: :4duckhunt:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
"Back in 1987, when Duck Hunt came out, these two might not have had the best relationship. That all seems to have changed now, and this unlikely pair are here to fight together."
It might be interesting in the case of some headcanons, to incorporate real life references like these.
(Note: Trophy descriptions are from the PAL version of SSB3DS, NTSC and Wii U descriptions may differ)