One thing I just noticed today, Celestia has the same plot idea thing as Dragonspyre. In Dragonspyre you have to get all 3 portal stones, just like in celestia, then you prepare a drake(a bit different in celestia, another portal) so you can fight the final boss. Has anyone else noticed this before, why do you think KingsIsle did it?
I think your example with the portals was somewhat coincidental. However, I also think that it has to do with the way KingsIsle structures the game's plot. In most worlds, the main goal is to finish off that final dungeon, or best the final boss. That said, there are multiple steps that you must take to get there. For example, you mentioned Dragonspyre's portal stones and the portal pieces in Celestia. Another good example is MooShu. When in MooShu, your ultimate goal is to heal Emperor Yoshihito from the curse of the Jade Oni. But, you must first kill the three preceding Onis to achieve this goal. In all, I think that all worlds have a different plot, but they have the same plot structure.