If you were with the empire than you could marry Elisif and become king. I also had an idea that during the siege of Solitude on the Stormcloak side, what if Ulfric was about to be killed and you had the option to save him or let him die and take his place.
Once you are king then whole loads of new quest opportunities opens up. If you became king by marrying Elisif perhaps there was an option to leave the Empire or vice versa with the Stormcloaks. What if the Falmer started terrorizing towns and became a real threat. Rebellions against your rule could happen randomly in cities and towns. You could appoint new Jarls from your followers. If you became king from the Stormcloak side, you could choose a queen (or maybe have Elisif killed and just grab a hotter wife XD).
Maybe I let my mind run away or I've been playing too much Mount and Blade but I think it would be a cool!