An ancient vampire should be equatable, IMO, to a Dremora Lord as suggested. I mean, what else can they be compared to? Certainly if you take, say, perhaps an Archmage of the former Mage Guild, who should be among the most powerful spell casters, versus a spell casting Ancient Vampire (leader of a clan or not), surely the Vampire would outpower them. Definitely they wouldn't be as immortal as the Dremora Lords, but of course they're at least partially Daedric, too. It'd be nice to, if you come across one, to get that eerie and overwhelming feeling of awe when fighting.
As for normal vampires, I think it should be only moderately harder to fight them. If to use Oblivion's Vampires as an example: The bonuses you got didn't really change anything. I cannot say how it would be in Skyrim, but, say, unless they haven't fed for a long time, they wouldn't be that strong. Now, if you're fighting them, then they probably are strong, for whatever reason, whether not feeding or something else.
So - they'd pack a slightly bigger punch (Or is it bite...? lol
) but every fight wouldn't be epic.
I definitely would love to see more "Ancient Vampire" stuff, compared to Morrowind and Oblivion. Of course TES isn't a game about Vampires, but... It is more or less a faction in and of itself, like the equivalents of Fighter and Mage (despite being disbanded). So having that side of gameplay fleshed out would be quite lovely.