Okay, here's one; if you HAD to give a second character to each third party character already existing in Smash Bros., who would you pick?
Sonic: Tails
Mega Man: Proto Man
Pac-Man: Blinky
Ryu: M. Bison
Cloud: Tifa
Bayonetta: I literally do not know the series well enough. I own 1 and 2 and fully plan on playing them sometime, so I've avoided anything that could be a potential spoiler.
Metal Gear: Raiden (not sure whether it counts or not, but I'm including it - of the series' three main characters, he's the most different from Snake)
Sonic: Eggman (I am honestly torn between Tails, Knuckles, and Eggman, but seeing as Eggman is most of the bosses in classic Sonic games, I feel like he'd have a much more creative moveset)
Mega Man: Zero (if spin-offs count. I don't care whether they go with MMX Zero or the Mega Man Zero version)
Pac-Man: Blinky (...there aren't a great deal of options here, but from what I recall, he's generally treated as the leader of the four ghosts)
Street Fighter: Chun-Li (there are a lot of options, with the main alternatives for me being Bison and Akuma, but Chun-Li definitely sticks out for her place in video game history)
Final Fantasy: Terra (from Final Fantasy 6 - I figure that she'd work well for demonstrating the series' magic system, since she's one of the few characters who can naturally learn magic in her game)
Bayonetta: Jeanne (they could go the easy route and make her a (semi-)clone, or use her to demonstrate the many, many,
many other weapons in the series. Rodin's a very close second, though)