This is something I have been thinking about for quiet a while now.
What if, the Dovahkiin, after he defeat Alduin, decides to take over Tamriel? I mean, we all know how powerful the greybeards are and that is after a life of constant training. What if the Dovahkiin, after Alduins defeat, went to the Greybeards and then one day, had such a powerful Thu'um that not even the Thalmor could stop him? Maybe the return of the Dovahkiin isn't such a good thing afterall.
There could be added alot more, but I will let it be up to you to decide if this could be a possible scenario. So what do you think?