The only other legendary Pokemon from Black/White that I think has a shot for Smash 4 is Genesect. Its the only legendary from that game not revealed yet, and its origin story makes it seem like a modern Mewtwo (basically, a Pokemon made by scientists create the world's strongest Pokemon).
I looked at the Genesect event for Black/White, and I think there is a good chance that there will be more to this Pokemon's story in Black/White 2.
I'm wary of Genesect due to how closely his situation mirrors Deoxys'. They're both legendary, multi-form, "scientific" Pokémon. I remember quite clearly that a lot of people included Deoxys in their Brawl prediction lists because they perceived him to be the Mewtwo of the third generation. I get a similar vibe from Genesect.