The dragon cult would also allow for greater dungeon variety, another element lacking in vanilla Skyrim, by adding in new and cleaned up dragon cult shrines, hideouts, forts, and etc. Since Akatosh and Kynareth are sacred to the dragons this could also cause a schism in the worship of the divines, which could add another cultural and political layer to the main story's conflict. A few more friendly dragons in the vein of Parthurnax would also vary things up a bit and give the dragons as a whole a bit more depth. They don't have to be good necessarily either and could just be jealous or resentful towards Alduin. There's a lot of room for expanding the main quest via this route as well, and it could be accomplished through the use of completely independent and new npcs and events to further flesh out the conflict. The hardest thing about this project is trying to be compatible with the existing main quest events, factions, and characters so more solutions like this might work. The dragon cult introduces the possibility of a secondary antagonist as well who could breathe new life into the quest line and they could play a completely different role and persona to that of alduin.