Please note, this is all speculation. Some of this is what I would like to see, and some of it is what I think we will see.

This has been my train of thought.

Dawnguard was Bethesda's first foray into Skyrim DLC. They wanted to experiment a little, and not do anything that would really have any impact on future lore. Hearthfire was just something to release between Dawnguard and Dragonborn- they knew they had to release something to keep the fans happy (of course, this worked better with some fans than others).

Dragonborn is a different animal entirely from both of the past two DLCs. I think it will be the first to act as a true expansion pack. I think it is meant to help Bethesda nail down the formula for the next- possibly final- Skyrim expansion. The theoretical name for it would have to be "Summerset". It involves travelling to the Summerset Isles/Alinor and planting the seed of war. It will pave the way for The Elder Scrolls VI, which will feature the Second Great War as the main plot line. Summerset will be the be-all end-all of Skyrim DLC.

The things I would like to see in Summerset are:

• New weapons and armor.
  
• In-depth, detailed cities.
  
• Interesting characters with a good amount of depth.
  
• Two different storylines, if there are two different sides to choose from (i.e Stormcloaks/Imperials vs Thalmor).
  
• A better balance of scripted moments vs. unscripted moments.

I think a Thalmor DLC is the only way to go after Dragonborn, considering how instrumental the Thalmor are to the Fourth Era.

I'd love to hear your thoughts on this. Just try not to say something like "oh well since Bethesda is a terrible company I'll never get anymore DLC anyway since I play on PS3". Keep it constructive and relevant.

The next dlc has to be TES:VI related.