Guest characters always manage to push many people's buttons, but what about non-MK backgrounds, whether we'd get the appropriate characters as guests or not? You could get many from WB properties alone.

Like the Overlook Hotel from the Shining, particularly the gold room, with all the classic music playing, the guests and characters like Delbert Grady and Lloyd the barman in the background. You could even add absent MK characters who'd randomly appear among the guests sometimes, all dressed for the occasion (imagine Moloch in a tux).

Or a cartoony Road Runner stage, with the Road Runner (duh) and Wile E. Coyote in the background. It could have a stage fatality where the loser gets killed by Wile E, or one where the losers realize they're standing on nothing, then fall to their death. Sometimes, we should see Kabal, and/or the Flash racing with the Road Runner.

I thought they wasted an opportunity by not including a ANOES stage in the last game. Sure, Freddy already had the boiler for one of his fatalities, but a ANOES stage wouldn't have needed it imo, as the fight should have taken place outside Freddy's house, with the creepy girls singing "one, two, Freddy's coming for you".

Finally, another non-MK stage I'd personally like is a NCIS background (I know NCIS isn't a WB property, by the way), where you would fight near the desks where most of the cast work, with the NCIS theme as it's, well, theme. They wouldn't even acknowledge the fighters, for the most part, but they could be used as projectiles. I wouldn't mind a minor stage interaction where Gibbs slap a character in the back of the head if they stand near him for too long.

You should have the option to turn guest backgrounds (and characters) off for single player, that way, purists can be spared the trauma of fighting in them, or fighting guest characters, or fighting guest characters in a guest background (that may not even be theirs, the horror!).