When you think of the new zones you can't really think of what'd be cool but would make HEREOS of abyssea really challenging. Small cramped zones don't seem like they'd be condusive to any NM battles. Honestly, would you want to kite something in a small little corridor in castle o?
My guesses.
Uleguerand Range(or Beaucedine Glacier)
Grauberg
Fort Karugo-Narugo
They're all vast areas with all types of terrain. You could easily fit the amount of mobs that usually go into abyssea plus empty space to fight the epic battles.