Well, they brought back THE Pit...kind of...so I won't vote for that one.
But I'd like to see the return of The Bell Tower from MK3
A version of The Living Forest with actual thought put into it. You should actually be fighting in the forest so you can knock your opponent into the trees, And they should actually move or something hence the name 'living forest.' rather than looking like a 'dead forest in the distance.'

^^ Exacty what I was going to say.

Yeah, it'd be really cool with a Living Forest stage that's similar to the forest stage in DOA3, where you could knock eachother into the trees and stuff...
Otherwise, I'd be happy to see pretty much any stage from the first MK's... but the ones I would want the most I guess would be Warrior Shrine, The Pit II, Wastelands, The Armory, The Belltower and The Graveyard. To be honest I don't remember a single stage from MK4, so I guess I wouldn't mind if we never saw those again. :P

I would really like to see the Armory stage returned from Mortal Kombat 2. I could imagine all the possible props you could interact with on that stage. Both characters could have a conglomeration of weapons to choose in picking up and using against the opponent. I remember there were two fountains of molten steel pouring into a little trench that leaked to another area. Imagine kicking the opponent into that part and having the molten steel pour on to their head and melt their flesh and bone? Another Death Trap idea would be incorporated with the Dragon Emblem that hangs on the wall in the backround. Imagine kicking the opponent into the wall and then they fall into little stream of motlen steel and watch them try to scramble out of it. The impact of the opponent would have the Dragon Emblem fall off the wall and collapse onto the opponent and crushing them.

The GraveYard (MK3).
The Armory (MK2).
The Streets/Subway (MK3).

that's kinda too much like the buddah stage, i'm very happy to see the deadpool make a come back...
the PITII ehh, too much like any other pit minus falling through wood platforms
the Graveyard would be a cool comeback i guess, with arms Coming out of the grave and tripping you, even have a grave at all 4 corners of the map to where someone pulls you into the grave with them as a death trap.
back when MKII was around, the first couple months I thought the meat hooks on the stage would be a pit fatality, just an idea i think would be cool for MK7. double death trap style