I agree with Jim on all counts!
The inherent necessity of the birth of a new goddess (imho) is that the universe *Needs* the new goddess. Remember, the workings of the universe are vast and unknowable I think that has to be spoken of first and foremost.
I think this need has to be worked into the basic aspects of the Test of the Starstone, for instance;
Cayden got drunk and can't remember how he did it ... but there was no patron that fulfilled his niche in the universe, so the question then becomes who dared him? Was it maybe a representative from the deities that realized an imbalance existed in the universe and then sought out someone that could fill that role? Maybe the patron even went to other lucky folks and gently nudged them towards the role, but some were too meek to attempt it, others tried but their luck didn't hold, etc.
Any adventure I would see as forcing a deficiency in the universe that the universe as a whole seeks to balance, then, through direct or indirect means, the PC's become embroiled. As they answer the call of the adventure they are drawn inexplicably into the Test, and at the conclusion of the adventure the one destined to fill the emptiness meets the goal. This could be an NPC who the PC's have been helping (a la Amieko from Jade Regent) or it might be the PC's. Wouldn't it be interesting if it were only one of the PC's? Maybe only one could meet the challenge, maybe they were there to challenge the antagonist and through doing so convert him to a protagonist, thereby maturing him into the candidate the universe needed ... so many possibilities!