In the upcoming expansion Dawnguard, just to clarify, there will be this;
-New quest line.
-More weapons (Crossbow is confirmed. Along with several other new weapons in the Dawnguard arsenal. Also Dragonbone weapons, which are reportedly stronger then Daedric.)
-More armor (Dawnguard armor and Vampire Lord armor, as seen in the trailers and gameplay footage)
-A new mount
-At least one new shout which IS the ability to summon an undead dragon, you learn this shout from the same dragon in the new realm.
-A new realm, The Soul Cairn.
-Armored trolls, which can be "Hired" to follow you and fight for you.
-A new race; Vampire Lord
-A transformation for the Vampire Lord race.
-A skill tree for Vampires and Werewolves, which will not use the traditional perk system, but will instead use kills and feedings.
-Two new factions, The Dawnguard (The "Good Guys" who fight the Vampires and Vampire Lords.) and the Vampire Lords (The "Bad Guys")
There will be new monsters, among these are several types of "Draugr" including wraith forms of them, which are stronger versions of the already existing ones.
Also, if you've gotten far enough in the game, you'll know that at Ancient Dragons, it gets pretty damn hard... Well, prepare to re-sharpen your swords and axes, because now you'll have "Legendary Dragons" stronger then any dragon seen before in Skyrim.
Have fun!! ^-^