The Questline for both Dawnguard and Vampires story paths has to be extremely great.
I agree it should be a 50+ hour expansion, and the expansion questlines should encompass meaningful long distance travels (I don't use fast travel, but even then Skyrim's questlines are too short. If the Skyrim guild questlines are short and somewhat anticlimactic, what can be expected from Dawnguard expansion? )
The vampire story should include prescripted npcs that you can bite and turn into vampires (to add to the family and the vampires plans.)
good rewards (like becoming new leader of the Vampires / new leader of the Dawnguard after the questline is completed).
good character interaction (not like Mages Guild where you can skip socializing with most of the Mages, each which had somewhat interesting quests of their own), in short the Dawnguard/Vampires questlines should make it mandatory to socialize with all the characters that have their own branched off quests, that way they can deepen and broaden the length and scope of the Expansion storylines.

After I completed the Mages Guild for example, I was surprised how much longer it could have been if I was approached by the Mages in the Guild that had their quests for me to do, and overall give more feeling of promotion and progression instead of breezing through the ranks without even being rewarded the proper garments for the new rank I aquired. Mages Guild questline felt like I went from apprentice to leader in no time. Really shallow feeling.
maybe the writers and questline designers will deliver a more than epic questline for Dawnguard and especially the Vampires which hold even more potential for a more refreshing and grand experience.
It's also good to have some well placed journals for the characters that feed the story and progression with greater purpose and insight.
It would also be great if that vampire fortress/castle is larger and complex than it looks. Well I'm not expecting a labyrinth inside, but it would be awesome if it had multiple floors underground that leads to the true lair of the vampires.

I think dungeon design as of late has been a tad too casual. Dungeons overall seem to have that "enter then exit" kind of feeling. Well there are a few exceptions like that deep Dwemer dungeon leading to the _______ in the SKYRIM main quest. But they really can make the Dawnguard expansion dungeons like really grand.
I don't think the expansion should just toss some horde waves of enemies type thing. maybe one event in the questline may have that, but the expansion shouldn't abuse that like some one trick pony.
overall I think key things that would make Dawnguard more than expansion would be:
* great travel distances Dawnguard questline encompasses (epic journeys = more epic questline)
* great travel distances Vampire questline encompasses (questline travels should encompass most/all of Skyrim)
* new characters that hold great presence (I think the vampires should have an order of main vampires. Well of course there would be a leader, but it would be cool to have like 6 or so other vampire characters that have involvement in meetings and missions).
*make sure to have female version model of Vampire Lords for Vampire Ladies transformation.
* some dungeons/locations that require torches / light / vision ability to see in the dark. Would be great if Vampires have a multi floor underground dungeon beneath the castle/fortress that's very dark, deep, open (many side rooms to explore), and awesome.
* well placed journals of key characters in the questlines that enhance the story, character depth and unveiling secrets.
* key characters getting killed in Dawnguard questline during heroic events. There should be some scary survival horror like events too.
* key characters being turned into vampires in Vampire questline to expand their influence in Skyrim, and also give player interactive moments of signature vampire nature. (could be animation triggered through a conversation choice event.)
* rewarding gradual advancement in the Dawnguard ranks. (with new gear each rank)
* rewarding gradual advancement in the Vampire ranks. (with new clothes each rank, new accessories and/or new abilities/perks)
* Very Rewarding questline completion ( new leader of Dawnguard, new leader of Vampires with new companions would be better than just some title/trinket/weapon/armor. Skyrim doesn't allow us to be Jarl of a city, or even The Companions for that matter since they aren't leader based, so let players be the leader of Dawnguard/Vampires at least. yes you can be leader in like the Mages Guild but the path to the top felt so tacked on and hollow. )
*Overall I think Dawnguard quests will naturally be about finding and eliminating vampires.
*While I think a big part of Vampire quests should be about
expanding their influence and numbers (turning people that are in high places into vampires, the player themselves especially should be the one that does that task) , besides exterminating the Dawnguard and finding items/clues to an Elder Scroll.

*Also the Vampire characters in the expansion should be united and loyal. There could be some backstabbing but I think there should be more backstabbing taking place within the Dawnguard than within the Vampires.
Betrayal only weakens a vampire questline/faction/story instead of improving it, especially considering in most stories out there, vampires end up losing. So vampires fighting among themselves would only result in a boring questline and would degrade the organization into a mere band of mercenaries and thieves. The Vampires should be the most 'noble' of the two factions.
I don't know if they have plans to expand the Lycanthrope content but I think that might get in the way of making the Dawnguard and Vampire questlines and content bigger and better. Werewolves/Lycans would need their own expansion to improve them properly.