Well, I think it just has to be something to do with the Thalmor for a start.

I think the setting should be either still in Skyrim, in new places (like how they added the forgotten vale in Dawnguard) or somewhere rather remote, like Solstheim (Atmora). I think the plot should be that you are exploring the place (as you do) or you get one of those Thalmor hit men has a note on him similar to how Dragonborn starts. You have to find an artefact of great magical power, or to be more precise, Dawn Magic. Depending on which side you choose/chose in the civil war you will be assisting them in retrieving this powerful object or tool. You will be fighting the Thalmor through various settings in the area which will eventually culminate in an epic battle between a Thalmor general, who is the antagonist, and yourself.

This artefact can also explain what the Synod was looking for, and it is what caused the void nights. It is then revealed that the Thalmor used this divine object to vanish the moons and then returned them so the khajiit would ally with them.

Now,I am terrible at writing down ideas, so if anybody can help me expand on this and add more detail please do!

I'm no creative person, but the Thalmor should really be addressed in a future DLC. That said, there should also be something for Winterhold. There is potential there.

I'm thinking a more dragonborn sized DLC

But it doesn't have to be outside of Skyrim.

I don't think a Big dlc would work inside of Skyrim. Too many places are here already we would need a new place. like Solestheim..