What is the main story line in skyrim going to be i know it has a lot to do with dragons but what will be the purpose
bliviongate:
From what we discussed in-depth about lore, it's likely to be this: the events of the four previous games messed up the dragon-god Akatosh (yeah, turns out mortals can do that) so now he's entered the mortal realm in the shape of Alduin, the World Eater to destroy it utterly. A prophecy has foretold that this would happen, and that there would be a mortal with the same ability as the dragons (the aptly named dragon-shouts) who will fight to prevent this. Namely you. Now, since he is pretty much the source of all time in the world, you can't just go and kill him. So you'll need to figure out how to turn him back into the good-willing Akatosh. That, or lock him up somewhere where he can't do harm.
This is set on the background of a civil war in Skyrim, sparked by the death of it's king and no succession. Each major city boasts a political faction seeking to secure rule over the war-torn country, hostilities between which have turned into all-out war.
The rest are details that the devs go to great lengths not to divulge.