Made a few tweaks to my list some of these probably stand no chance of showing up, but i'd love to be proven wrong, I do atleast wish we knew what the final count was for the roster, then it might be easier to narrow how many to expect, and then make more realistic guesses. Also the characters already announced are not listed, because we know they are in.
Battle Royal Roster wishlist:
Abe (Abe's Odyessy, Abe's Exodus)
Doll Face (twisted Metal) really they could add pretty much anyone from TM
Sackboy, with a Sackgirl alternate skin (LittleBig Planet)
Lammy (Um-Jammer Lammy)
Daniel Fortesque (Medievil)
Jack (Jack and Daxter)
Daxter (Jack and Daxter)
Rachet (Rachet and Clank)
Clank (Ratchet and Clank)
Angela Cross (Ratchet and Clank)
Dart (Legend of Dragoon)
Rose (Legend of Dragoon)
Maximo (Maximo vs the Army of Zin)
Drake (Uncharted)
Elena (Uncharted)
Sev (Killzone)
Dante (Devil May Cry)
Lara Croft (Tomb Raider)
Solid Snake (Metal Gear Solid)
Meryl (Metal Gear Solid)
Revolver Ocelot (Metal Gear Solid)
Chris Redfield (Resident Evil)
Jill Valintine (Resident Evil)
Berry Burton *Preferred* (Resident Evil)
Brad Vickers (Resident Evil)
Cloud Strife (Final Fantasy VII)
Aerith, or Tifa (Final Fantasy VII)
Sephiroth (Final Fantasy VII)
Wanderer (Shadow of The Colossus)
A Colossus (Shadow of The Colossus)
Fei Fong Wong (Xeno Gears)
Kazuma Kiyru (Yakuza)
9-Ball (Armored Core)
Alundra (Alundra)
Nomad (Journey)
Kazua Mishima, (Tekken)
Eiji Shinjo (Battle Arena Toshinden)
Robbit (Jumping flash)
Crash Bandicoot (Crash Bandicoot)
Spyro (Spyro the Dragon)
Specter (Ape Escape)
Klonoa ( Klonoa Door to Phatomile)
Alucard (Symphony of the Night)
Aya Brea (Parasite Eve)
Levant (Jade Cocoon)
Kain (Legacy of Kain: Blood Omen)
Suezo (Monster Rancher)
David Ortiz (MLB the Show, first cover athlete)
With the exception of a few just crazy ones like Jade Cocoon, or Monster Rancher characters, I think much of my list is reasonable, if they make Abe playable though this game will be a day one no matter what.
For some stage mashups they could have a space stage that combines Killzone and Colony Wars, they could have a GT/ Whipeout cross over stage, or for a singular stage they could have a stage for Uncharted 2 where your in building that is periodically collapsing around you.
I was thinking that if they have stages for third party characters then I would think those stages might not get mashups, although I guess it would be possible to mash sequels together. An example would be mashing FFVII/FFVIII/and FFIX, but I doubt S-E would allow something like that, but it's also advertising their games aswell so it would be a silly thing to fight really.