Flyboy
Smash Hero
Even Bayonetta's stage is literally the opening sequence from the first game.If the Shadow Moses stage spoiled MGS4, it was in a subtle way - Shadow Moses hadn't even been announced as returning in MGS4 when the Smash Bros. stage was made, (the announcement video for that reveal included Smash footage) and the inclusion of other Metal Gears could easily be seen as a fun easter egg, or a way to add some variety to the background.
There's a big difference between that and including the final area of a lengthy RPG as a Smash Bros. stage (I haven't finished Persona 5 yet, so I don't know the stage or its significance) - Smash tends to lean towards early game material with its third party stages (Cloud got Midgar, the first area in FF7; all of Sonic's stages have been intro stages), with Mega Man being the only exception that I can think of.
I mean, it's a fantastic choice for a stage, but still. It's part of why people were predicting the Casino as Joker's stage - it's a beautiful stage and the opening that sets the tone for the game. If Mementos is the stage, it'd likely be one of the subway stations and MAYBE the first Palace.