Sony has plenty of characters to put into a cross over game similar to Smash Bros.
Kratos, Sweet Tooth, Sackboy/girl, Jak, Daxter, Ratchet, Clank, Sly, Bently, Murray, Normal/Infected Nathan Hale, Nathan Drake, Young Nate, Kathrine Marlowe, Elena Fisher, Zoran Lavarevic, Chimera, Jan Templar, Sev, Dart (LoD), Rose (LoD), Parappa, Patapon Knights, LocoRoco blobs, Toan (Dark Cloud), Colonel Flag, Dark Genie, Max (Dark Cloud 2), Monica, Emperor Griffen, Leonard (WKC), Yulie, Princess Cisna, Cole Mcgrath, Free Realms Squirrel (lol), Hell even DC characters could be involved in some way (Batman!). Just name the characters I would like to see from Sony developed games.
Let's not forget that Sony also has a powerful relationship with Namco (having published or co-published numerous Namco titles outside of Japan). Final Fantasy VII and X were under Sony ownership.
All in all Sony can pull in third party characters to fill in potential holes in the roster. Auron, Tidus, Kimahri, Bahamut, Alexander, Ifrit, Shiva, Lulu, Cloud, Tifa, Aerith, Barret and of course Sephiroth. How about adding Sora? Donald Duck? Goofy? Mickey?