I don't believe that after making Dawnguard all Elder Scrolls employees (those not currently working on MMORPG) spent all their time making Heartfire. So... how large and impressive will be next DLCs? How possible it is that they will add more teritory to explore, large amount of quests or something else very impressive?

http://www.gamesas.com/topic/1417703-18-beta-update-solstheim-dlc-hints-thread-15/ series of threads holds some interesting speculation on the next DLC.

Haven't you heard of the rumors of the next DLC? There's a thread right http://www.gamesas.com/topic/1417703-18-beta-update-solstheim-dlc-hints-thread-15/ about it.

Edit: DAMN YOU ARCONA Ninja'd .

Not as impressive as Bloodmoon. That's for sure.

Well that depends... What periodic attacks it's going to include?

Why was Bloodmoon so impressive?

Well, considering the answer to such a question resides in the realm of speculation and personal opinion, one of which cannot be fully realized without first experiencing it first hand, I shall give my response in brief...

How the [censored] should we know?

In my opinion?
First time an enemy ever scared me (Spriggans back then had to be killed thrice before it stayed dead, I ran for the hills the first time it got back up).
Gave you the opportunity to go mano-e-mano with an aspect of a Daedric Prince.
A location that featured forests and snow, as opposed to Morrowinds drab deserts and swampy locales...
Things like that.