My theory on the OGB is that it will try to usurp the throne. If Alistair and the female Warden have been chosen it has been made pretty clear that they can't procreate due to the fact two Wardens can't have children. If Anora and a male Warden have been chosen it has been rumored that Anora is barren. Furthermore, if Alistair did the rite with Morrigan, the OGB has legitimate claim to the throne. If Logain and Morrigain ended up doing the rite and Anora is the ruler, the baby still may have legitimate claim if Anora actually is barren and Anora is "disposed" of. If the Warden did the ritual with Morrigan and is now ruling, well thats obvious.
Although Morrigan said she would have no desire for claims to the throne, that doesn't mean OGB wouldn't want claim of Ferelden.
Any angry female Wardens stab Morrigan?

(LOL) Another reason for OGB to want to step in and steal the throne.
Any thoughts?