So, I'm trying to understand Mega-Man a bit more, and I'd like to know what makes him so likely? I keep hearing everyone saying he's oh so likely, but I don't see why.
Well, to begin with, he is overall one of the most requested characters in general and the most requested 3rd party character. This much is undebatable. However, unlike many other characters, he has a large amount of popularity with pretty much every demographic, other than the Nintendo purists, which is quite important to Sakurai. He is requested by hardcore Smashers and casual Smashers, by younger gamers who've played his more recent (but not that recent -- dammit Capcom) games and older or retro gamers who remember his 8/16 bit days. He's requested heavily by the people here on Smashboards to the people on GameFAQs, the ones on IGN and even the fools on Youtube. Even random occasional casual Smashers who don't have accounts on anything and aren't familiar with half the roster usually want Megaman.
He also has high worldwide popularity, as opposed to characters like Ridley, Little Mac, and Isaac, and in fact in Japan he is overwhelmingly requested (moreso even than in the west, on average) and he is the most requested new character for Smash 4 there (not counting Mewtwo and Roy -- they're not new), as well as being one of the top three here.
Secondly, and this is far from a guarantee, but Sakurai did say that a Capcom character could be a possibility. There is no doubt that Sakurai is aware of Megaman's immense popularity, and if he chooses to persue new 3rd parties (which IMO is probable, considering he recently spoke positively of them, as well as them generating massive hype and helping with the sales), then it would be sensible that he would try to start at the top. Even with Inafune leaving Capcom, they seem more than willing to allow Megaman be included in Smash 4, with both Capcom USA (for whatever that's worth) and Ono vouching for his possible inclusion.
Assuming Sakurai would try for the most popular 3rd party, it seems like very few other obstacles stand in the way of Megaman, as Capcom seems very open to him in Smash. While granted, he doesn't have the same amount of popularity Sonic did, he is still far and away the most requested character overall (counting all demographics and all regions), and logically if Sakurai was still for 3rd parties it would be likely he would make whatever legal efforts necessary to obtain the newcomer that would bring the most attention and acclaim to the game.
Now I'm not saying that other 3rd parties aren't going to happen because of Megaman (I'm not even saying Megaman is a lock -- he's definitely not, even with his good chances), just that even with Namco working on Smash, I believe Megaman will get top priority in 3rd party inclusion. Mixed with the fact that Namco firstly doesn't seem to be pushing Sakurai for character content, rather letting him decide, and secondly, so far not giving a clear character choice (no one has even mentioned Pac-Man yet, though Tekken and Tales have been mentioned). So far Sakurai has made no comment towards a Namco character, so the chances of one are still relatively up in the air, and it would come down to his decision. And even then, it's no guarantee he would pick someone like Pac-Man (though that would be the best choice). So while a Namco character still has a decent shot (as Sakurai might want to "thank" Namco by including a character) and it very well might be Pac-Man (that's who I'd pick for a Namco rep), I still believe that the much more requested Megaman has the better chance.
Phew, sorry the response took a little while asage.