I actually am beginning to doubt another Fire Emblem character. I have read some of your posts insinuating that you believe 50 characters to be the magic number, correct? I feel the same exact way. If Sakurai asserts that 39 (40 including Mewtwo, and for the sake of this discussion, I will allude to Mewtwo as if he is already in the game). Thus a 25% increase in the 4th installment of Smash is completely and utterly logical, that is a 10 newcomer increase counting the 4 we already have revealed. Again, this is also presuming that the roster is to retain no omissions. That leaves us with 6 characters and a dozen franchises that can be represented.
I believe everyone agrees that we will get an additional representative from Donkey Kong, Kid Icarus, and one of the Big 4 (Mario, Legend of Zelda, Pokemon, Metroid). That only leaves us with 3 more characters, needless to say, despite many proclaiming that series like Star Fox, Kirby, Mother, F-Zero, WarioWare, Yoshi, "Wii," Animal Crossing, and 3rd party characters are "complete enough" or "finished." Sakurai might not feel the same way, it would be blatantly fallacious to disregard the feasibility of one more character from any given series already on the roster. Could that character be from Fire Emblem? Sure. Again, Fire Emblem isn't necessarily saturated with diverse characters that offer a lot in terms of variety.
We still have to account for a retro character, at least one is essentially guaranteed (so we have 4 slots that aren't Fire Emblem). After that, we have to account for new series representation such as Shulk or Isaac... maybe even another character that no one expects like Wonder Red (so we have 5 slots that aren't Fire Emblem, assuming we get a new series). Lastly, we go back to paragraph 2, there is only enough room for one character out of 9 categories, and I probably missed one or two (this is where Fire Emblem has a chance).
If Sakurai is focusing on roster variety, Fire Emblem isn't necessarily going to take priority over other characters unless they pick someone like Robin or a low priority clone that is easy to develop.
Again, he might go above 50 character, and this assessment doesn't even account for double representation. A real possibility if we get characters like K. Rool and Dixie or Toon Zelda.