I think the third party lineup we've seen in Smash are pretty well-deserved to be there. Sonic, Mega Man, Solid Snake. Then there are a token couple of the early Japanese game developers left for there to choose from, most notably Square Enix and Bandai Namco. Some other prominent studios could be Tecmo Koei, Level-5, and even Platnium (they seem to be the rising stars. Like a potential next-gen Capcom.) All of which have a strong relationship with Nintendo and have the rights to some of the greatest games and franchises from the gamer to the everyman.
Although, I would be concerned about tiering "legendary" characters and their priority over characters deemed "iconic". A drop in the status attributed to the current guest lineup is convincing and will likely reach by the next few installments (from that point we'll have seen a plethora of new games, characters, and industry developments.) It just seems unfair to rank the likes of many qualified 3rd parties from what I see as a cocktail of nostalgia, relevancy, sales, pioneersmanship, etc. I'd argue if Solid Snake is even a legend. Because if we're talking about Sonic or Pac-Man levels of recognizablity, I would have heard about MGS pre-Brawl. Yes, I say that sincerely. Not to say I don't like Solid Snake and he's always stood out amongst many other video game mascots. But what does it say for other characters? The majority of 3rd parties I frequently see being supported can get in on their own merits. Just as long as Sakurai's standards deem them so. He is a very picky man.
Anyways, you know who I would love to see? NiGHTS. Another classic gaming icon that shines out past the uncertainty of the Sega Dreamcast days. I can imagine the wealth of material you could incorporate into a unique moveset. Hell, NiGHTS into Dreams just feels great to control. I can't think of another game of its genre! If only it caught on, NiGHTS could have pioneered a genre, as loose as one may ascribe an action-adventure label to it (which I think doesn't describe the gameplay well enough.) Sonic can't necessarily claim that easily. To curry the favor to Nintendo even more, the sequel was a Wii exclusive along with cameos wrought over from the Sega All-Stars and Sonic series. Although, I'm not sure if NiGHTS stands as much of a chance within its own company and other competitors. I mean, we already got Sonic twice!
Jack Frost from Shin Megami Tensei is very much deserving as well. He could have came in from how prosperous the Shin Megami Tensei series and its spinoffs are. But due to financial issues with Atlus, they are now shrouded under Sega's wings. It's ultimately for the better though, as Sega has been giving Atlus a great treatment even to the West mainly due to Persona.
Either way, NiGHTS and Jack Frost are definitely iconic enough to stand a chance. But really, what do you think about them? We don't want to clutter up the sega representation now, do we?