I just killed Harkon, after playing through the vampire side of Dawnguard. I'm just wondering if that's the end, or if there's more that I'm missing? Can I upgrade/refurbish Castle Volkihar? Can I get access to new equipment or other rewards (powers, spells, blood potions, etc.)? Are there any additional sidequests, maybe from the other residents of the castle? Can I do anything more with Serana and/or her mother, or maybe the Snow Elves? The ending just seemed kind of abrupt; I haven't actually resolved anything with the Dawnguard, and I don't see how killing Harkon really changes anything in the grand scheme of things. Where do I go from here?

You can bring serana's mom back, and there is some side quests

Thanks. I just went back to the Soul Cairn to pick her up (and also dropped off that horse's skull in the process). The only other side quest I've found so far is the one from Jiub (best easter egg ever).

You can do some radiant quests for some blood potions, chalice of the ancients upgrades and artifacts to super buff your character in normal and VL form

One of the radiant quests has you settle things with the leaders of the dawnguard..

Thanks for the info. Can you point me in the direction of which NPC(s) to talk to to begin these quests? I've tried everyone I could see in my quick run through the castle, but no one's given me any type of quest.

You have to do several other radiant quests for the Vampires beforehand. Eventually you will recieve one in which your objective is to take out the leaders of the dawnguard. it varies person to person how many quests you need to do before this one is offered, but it will most likely be one of the last sidequests in the VL questline.

Can you bring her back if you picked the Dawnguard side?