I would love to see Scorpion make an appearance.

I doubt it, as it is a PLAYSTATION All Stars Battle Royal, and Mortal Kombat isn't Playstation exclusive.

However, should an MK character guest in the game, then it would probably be someone boring like Scorpion or sub-Zero, as always.

Considering that Mortal Kombat has been more in the PlayStation camp lately, it's a significant fighting game of PS3 and that Snake made it to Super Smash Bros. – it's possible. Why not even two characters if they'll get the licenses for one character. But the other shouldn't be the the other of Scorpion and Sub-Zero seeing the "first" will likely be either, but "the other" should be someone else from MK1 minus Kano plus Mileena.

I'd rather see either Kitana, Sindel, or Reptile appear if they did bring in guest characters; Scorpion/Sub Zero are really getting old, it's time others shine.

This is possible, Sony have said that there will be 3rd party characters.

In my mind the front runners for this are Scorpy, Subby and Raiden.

This. Sub, Scorp, or Raiden are definitely the first characters who come to mind.

I agree that Sub-zero, Scorpion, and Raiden are the from runners and if any character gets in I honestly think it'll be Raiden, but I personally would like if my namesake made it in, I think cage could be a good add.

Considering that MK developers are working on the game and Sony and NRS apparently have a great relationship, an MK character appearing in Sony Smash Bros. isn't out of the question. It'd be really cool to see Sub-Zero or Scorpion duke it out with Parrapa the Rapper

Crash Bandicoot vs. Scorpion lol

Nathan Drake vs. Sub-zero

Kratos vs. Raiden

This game has alot of potential for DLC (3rd party characters are confirmed)

I wouldnt be suprised if it got like 10 character packs.

MK, Street Figher, Tekken, hell even a Sonic pack.

This