These are my suggestions

Lets start with the Akaviri suggestion.

When you have downloaded the DLC You see a boat at the Solitude docks and get in it but suddently the boat sinks in the mid of the sea near Akavir and you swim to a island where you get arrested and taken to a prison where they will look at you and say He is the Dragonborn he will kill all the dragons who almost killed all of our people! Then you do various of missions etc...

Now Dawnguard.

You meet a redguard in Whiterun who will tell you about the Aldermi Dominion trying to steal Hammerfell and you must help him and his country.

Now the one i really want!! If we can get this to be a DLC i will buy all Elder Scrolls games in the future and do everything for them!!!: Return of the Dwemer!

When you have downloaded the DLC you can see a portal in the College of Winterhold they tell you to get in the portal to check it out. When you get in it you will see a dwemer greeting you and telling you that they have been banished to the Daedra you will accept to help them and you start a quest called: Return of the Dwemer Where you got to kill the Daedra and get the Dwemer back to nirn. When you got them back the old Dwemer Ruins get to be towns like the others and will have citizens it will have some humans and some dwemer. They will also make new towns around the world and will have falmers in the basement to torture. You can also do a handfull of quests for the Dwemer.