The first row seems pretty filled out. I can't think of anybody the Mario series is missing at all, besides maybe Toad (please no please no please no)
Looking at the second row, it seems pretty unlikely that they'll receive more than 1 more rep (that one being Dixie Kong). LoZ is kind of at a dead end as far as main characters goes, but they could possibly do a surprise Demise/Ganon/Vaati appearance, but I think Sakurai will have a heart attack if he has to introduce more villains.
As for the third row, KI seems the most likely to get more reps, as Magnus, Phosphora, and even Pyyrhon could all be viable characters (at least moreso than Dark Pit). Kirby franchise would have one hell of a time fitting in something else; my guess would actually be Blade/Sword Knight, as Bandana Dee seems a bit too...passive, if you will. FE seems the most unlikely to get any more characters-Tiki and Naga have Ridley syndrome, Lyn is an AT, and Roy...yeah. If anything, Eliwood would probably fill this space. EDIT: omfg I forgot Anna (/shame); assume she'll be there over Eliwood if FE does get a rep
On the fourth row, Star Fox is almost guaranteed at least one more character, most likely Wolf. Krystal is also a possibility, but that honestly feels more like wishful thinking ;_;. Pokemon might get 1 more rep-Mewtwo, Blaziken, Zoroark, Gallade, and just about every popular bipedal Pokemon could fill the slot.
The fifth row seems like the "Miscellaneous" row, so if there would be additions, it would likely be from other games. Keep in mind the Miis have to fit somewhere, so it's likely either this row or the last one. If they do put one more character in to that row, my guess (hope?) would be Ice Climber(s).
The sixth row will either stay the same, have the addition of the Mii fighters, or randomly get a ton of third-party characters, although at this point it would be weird things like Nightmare and Snake. There's also the possibility row 5 will flood in to row 6, with the clones + Mii fighters filling out the bottom row.
tl;dr: I'd say Wolf, a Pokemon (Mewtwo?), Ice Climber(s), and a Kid Icarus character or Krystal are most likely to be the final characters.