After you've completed the Civil War and Dragon campaigns I would love to do preparations for a war against the Admeri:
Fortifying Skyrim against invasion by organising building defences, restocking and garrisoning forts, making diplomacy trips between Jarls to get them to work together, seeking out spys and informants, and mobilising the Companions, College of Winterhold, and even the Dark Brotherhood and the Forsworn depending on the quests you did.
If you sided with the Imperials you would get a lot more resources, to stock up forts, build defences, manage road blocks and checkpoints etc. although you would recieve less men, as many Nords would be drafted away to fight elsewere in Tamriel, but if you sided with the Stormcloaks you would get more men (mainly Nord forces, whereas the Empires forces would be more mixed) but less resources to divide between them.
The differnce would be that you would have enough resources to take back all the forts, fortify all the towns, build a pile of forward camps etc. but not enough men to guard them all, whereas with the Stormcloaks you would have enough men to garrison everywhere, but not enough time or resources to achieve it.
There would be an invisible countdown to an invasion (lasting a few months) before which several Aldmeri skirmishes would happen, increasing in rapidity to signal when the main force was coming, then they would attack at from several random areas at once (hitting Solitude or Windhelm from the sea, or coming over the mountains into the other holds.) You would then have to defend cities from the attackers, it would be arranged so you couldn't hold them all; some would fall while you defended others, and you'd have to keep fighting until either all the Jarls were killed and defeated, or you drove the attackers out of every city and chased the retreating forces back over the border.
I understand it would be pretty much impossible to have large scale invasions like this due to computing power, but a man can dream.