A Sony nostalgia-fest brawler without Tir McDohl would be a snorefest brawler indeed.
For all the constant reminders that Nintendo milks all of its first-party licenses for all they're worth, Smash Bros. was ingenious. Why? Because milking every property they owned ensured these characters would not be forgotten. Sony backs a lot of franchises, sure, but only until they lose relevance. They never try to KEEP them relevant. So while Smash Bros. has a couple characters in its roster that are a bit more obscure, most of its characters are immediately recognizable. Even adding in Snake and Sonic, who are very recognizable, is a legitimate move (the very first Metal Gear, long before MGS, was on the NES. And every current console currently owns a piece of Sonic now).
But what games does Sony have, and if not games, what characters? If they try to fish up every character that put them on the map, they're going to be paying out a lot of royalties, because that largely came from 3rd Person Parties. Tomb Raider(once Eidos, not sure who now), Jak n' Daxter (Naughty Dog), Crash Bandicoot (used to be Naughty Dog), Ratchet and Clank (Insomniac), Metal Gear Solid (Konami), Disgaea (Atlus), Suikoden (Konami), Katamri Damacy (Namco), SMT/Persona (Atlus), Ico, Shadow of the Colossus, Parappa the Rapper, Twisted Metal, Legend of Dragoon, Final Fantasy 7-13 (Square), Spyro the Dragon (once Insomniac, now ActiVision) Jet Moto, The Mark of Kri (which is at least partly owned by Capcom), Xenosaga (Namco), Tekken (Namco), MediEvil, Little Big Planet, Uncharted (Naughty Dog)... and that's only factoring in those things largely exclusive to Playstation outlets (which I say very loosely).
The licensing for those non-Sony-owned things alone will be legendarily horrific, leaving them with very few things in that list that they don't have to ask/pay for up front. Then there's all the games that used to be Sony-only but went cross-platform, such as the new Prince of Persia and the Resident Evil series. Then there's all the games that started out cross-platform. And Nintendo (who will treat this as competition) and Microsoft will be even bigger pains to pay licensing for.
Then there's the tone of the game. Smash Bros. is mild, cartoon violence. Even with grittier characters like Snake in it (and MGS is chock full of bizarre comedic moments), it's a lighthearted game at its core. Trying to combine the tones of all of the games Sony can choose from into a cohesive tone is going to turn out worse than a kindergartener's arts and crafts project. It's going to look and feel like a mess. Except there will be no soccer moms around to give it false praise. Well, maybe IGN....
Sony would be better off negotiating with Microsoft and Nintendo to do one big fanservice brawler using all of the characters between them. That's how you muscle in on SSB; you make a game that assimilates it. And you have Capcom's MvC team create the fighting engine.