So after having the better part of a day to mull it over, here is my roster prediction (assuming 15+4 DLC and no OC):
Fire Emblem
1. Marth
2. Caeda
3. Tiki (could be Awakening version instead)
Awakening
4. Chrom
5. Robin
6. Lucina
7. Tharja
Fates
8. Corrin
9. Azura
10. Xander
11. Ryoma
12-14. A villain from each of the game's...Although not sure which
General
15. Anna
DLC
1. Ike
2. One of the shape changers from either Fates or Ike's games
3. Hector (this might just be wishful thinking because I think he could have a great moveset based on his legendary weapon...Also there is a lack of axe users in my char list)
4. Roy
How much can I realistically hope for playable Nephenee in this release?
Uhh I'd go with 0%. She is probably a mid tier pick from a game that has very low chances of getting any characters in the first release. Best you might be able to hope for would be if they include create a character that has appearance options similar to her.
Problem with this game is no matter how large the roster is, there will be able to be enough character slots for everyone's favorites. One way to solve it would be to have generic movesets for all the major archetypes and robust create a character...but the number of styles they would have to make would basically be a full roster on its own (off the top of my head swordmaster, hero, berserker, soldier, mage, sniper, bow knight, Pegasus knight, wyvern knight, sage, dark mage, assassin, paladin, and general and that is without getting into Fates newly introduced ones). So I am going to be very curious as to how the hell they are going to try and solve this.