I guess it's possible. I think Rhalgr would be more intense/interesting overall though - and it ties in more neatly with Ala Mhigo. A large portion of 2.0's - and 1.0's - story revolved about Bahamut. 3.0 was similarly and understandably dominated by Nidhogg. It feels a bit too soon to introduce another major dragon that we have to fight.