I really can't take credit for all these theories! I've been avidly following all the wonderful meta by lonewytch and janie_aire and elisi and others - this is a synthesis of things I've read on LJ/DW, seen around on other sites, and things I've noticed myself! I think lonewytch and elisi are originally responsible for the 'egg' theory - I really liked it because I had already theorised that there will be a New Gallifrey soon, and this helped to explain how!

River HAS to be pivotal, surely. It's the only thing that makes sense! (apart from a musical episode, obviously :-P)