When discussing 3rd parties there is so much to take into consideration. Like Sakurai's view on them, as well as nintendo's view itself. Take overrepresentation into play when suggesting *insert sonic character here* because it's just unlikely. However I see another Sonic character getting in before another Metal Gear, if that's any conslation.
The way I see it is that there are TWO polar-opposite roads that SSB could take when we talk about third parties.
Road one would be that Nintendo and Sakurai see that potentially more third parties could bring in more fans, which could bring in more customers for not only brawl but all the particapating franchises and blahblahblah. You see where I'm going? If this is true, what we should be discussing is which conpany/ies are nintendo's most likely targets. Or which company is most likely to reach out to nintendo. I would take both things into consideration. Suda51, Square Enix, and Capcom are most likely
Reasoning for the three companies/designers: SE, and nintendo are thought to be close... but who really knows? I mean so many rumors swirl around the net about these two companies. Capcom we know is more than willing to let Megaman Cross over. Lastly Suda hinted he would like to see that guy from No more heroe in SSB. You could easily assume any sane comapny would love to have there character in SSB4.
Road two would be Sakurai having some sort of bad taste in his mouth about Sonic coming in, maybe ruining his original idea plan for the roster, or something like that. Maybe he wants this to be about "Nintendo All-Stars" and not about "nintendo all stars....and friends". Who knows what his problem would be, but lets say he has a problem. In this scenario I could see both Sonic and Snake leaving. Which would be a blessing to some, and would be a message sent from Medusa herself.
Personally, Either roads is fine by me. I HATE the fact that we have two third parties, I either want One to two new thir parties, or take them all away. I dislike Sonic games, and haven't ever played a Metal Gear game, so having these two leaves a bitter flavor in my mouth. Maybe Cranberry Flavor?
Edit: I don't see Smash just Keeping Sonic and Snake. I don't see things just staying
the same in the next game.
You get Chun-Li too sometimes. I'd say she's more than worthy for being on the roster (considering that she's appeared on every Capcom VS title to date). She just has Ryu and Megaman ahead of her.
If this makes her a worthy character, then Morrigan is Equally worthy!