When Sakurai said "I don't just add any 3rd party..." he was remarking on Mega Man's unique feel. It really doesn't discount any other 3rd party (2nd if Nintendo purchases Atlus) character. It's almost like he's telling the fans to calm down with some of the ridiculous choices they make.
I'm rooting for Jack Frost, as if you couldn't tell by my current avatar. But the only real way I see inclusion even more valid is if Nintendo purchases Atlus. Besides, Nintendo has been giving a lot of support to them lately with SMTIV as a 3DS exclusive, as well as Etrian Oddysey V. Not to mention the upcoming SMTxFE.
Roster fully finalized? With a grain of salt unless we see another Sonic situation.