This would be a nice idea, though it kinda restricts the plot development... If Bethesda decides to make a mainquest where you choose which path you want to follow, I'd like to have something like this:
-First you can choose any race you can, it will not affect the mainquest, so during the quest you must decide what to side to:
>Side with Alduin believing he will bring the "salvation". (Somehow grey path:)
>Side with Alduin to destroy the world. (Greedy/Evil path, maybe Alduin offers something in exchange)
>Side with Dovahkiin to stop Alduin. (Good path)
>Side with Mer to destroy men for a greater good (Selfish path)
>Side with the religious men to convince everyone that this is salvation. (Somehow grey path)
>Side with the non-religious men to convince everyone to stop Alduin (Somehow grey path)
>Convince Alduin of his madness and turn him into Akatosh again (Grey path)