You can take the major raider groups as Mercs too I'd think. It's something they would do. Vipers, Khans, Jackels to name the big three in the west. Talon would be the major ones in the eastern most portion of the map. In the midwest you can have the Reaver movement, Super mutant remnants, and generic Raiders, ghouls, and so forth.
I can only fit so many factions.
Speaking of ghouls, how will they be pictured? Will there be any kind of ghoul faction, or ghoul units you can train? Will there be a Necropolis, or other ghoul settlements?
They will be a Rebel Faction, though you will be able to recruit Ghoul Units