I would give it a 20 percent chance honestly, which I think is a lot lower then other people think. On one hand, if advertising the game was such a big deal to Nintendo and Sakurai, they would have added a character before the game came out. Not because of stupid patterns, but because that is always how you do advertisements (for example, Corrin was added before fates was released in the West, Roy was added before his game was released and Hero was added months before DQ11 S dropped). And to top that off, FP1 was entirely (assuming FP5 is the same as the others) unrepped third parties that have a long legacy. So on that note, I think WHAT the DLC is matters a ton for their chances. If its another FP and the first 2 fighters are unrepped third parties, yeah I would say their chances go down quite significantly and I would put them at a 5 percent shot. If its DLC being sold separately from each other, then I would say about a 25 percent chance whether Sakurai decides to add them. Its also is worth noting that the last time Corrin was added, the devs went to Sakurai to convince him, so he might decide not to choose a Fire emblem rep again if he isn't approached by the devs.
As for the rep, I think it would be Byleth, no question. Sakurai always picks the protagonists of the game, and there no doubt that Byleth is the main protagonist of the game, as all of the routes focus on him and HE is the main controllable unit. This isn't even a case like Awakening where you could argue Chrom was the main character, Byleth straight up is the main character. Although Edelgard would be cool and could be chosen, I just think it would definitly be Byleth.