Lets see... my predictions.
Since it's a Direct, looking at the characters who WERE Revealed in a Standard Direct. We're looking at:
(Might be missing some) But we're looking at a pretty even split of Nintendo/Third Party. But also for the most part Family Friendly Characters with Appeal thats generally broad (Aside from Terry) and not Japan Exclusive when it comes to popularity.
So with that and thinking the next fighter isn't a Swordsman or a Villain, I'm guessing....
1. Crash Bandicoot
2. Pokemon Rep
3. Rayman
4. Zelda Rep
5. Xenoblade Rep
If its a Pokemon/Zelda Rep. I could easily see them doing an ARMS thing again with a list of viable fighters.