There will certainly be some sort of expansion dealing with the Synod, as the Synod mage you meet during the College of Winterhold questline claims that the player hasn't heard the last of his organisation. I suspect this will simply take place in Skyrim.
Additionally, and call it a hunch, but given Skyrim's proximity to the island of Solstheim and the fact that it's an island that has a strong relation to werewolves [http://uesp.net/wiki/Bloodmoon], I wouldn't be surprised to see a Companions-related DLC taking place on the island.
And for the record? I rather doubt we're going to be traveling to a plane of Oblivion for a major piece of DLC as we did for Shivering Isles. Most of the prince's realms are either uninhabitable by humans or simply unfeasible locations for a game, and the remaining ones are ruled by princes either too powerful to have need of a champion or too boring to make for a decent story. I suppose there is a chance of a trip to one of Nocturnal's realm if they do DLC for each of the four important factions, but I think it highly unlikely.
