you don't need morrowind to enjoy the game, and new DLC's are stand alone (though the story is very slightly connected, but its not necessary to play one, to enjoy the other)
and yeah there is an event that's going on right now, for 2 more days. do a random dungeon - get a box with stuff. its mostly crap, but occasionally you might get one of 4 costumes, an imperial horse, puppy, some new DLC's furnishing patterns and if you are not a subscriber, dye stamps are nice, i suppose. I am at this point convinced that jackpot is a criptid. (jackpot is supposed to be a collection of 20 rare crown mounts, or 20 houses, including those limited time crown only houses they had on the market)
rewards are tied to premium undaunted exploration supplies, so there's 20 hour CD per character. so far, I personaly got 2 costumes, 3 clockwork furnishing patterns one of which I already knew and the rest - crap. but... costumes are nice and I'm getting ridiculous amounts of tanking practice, so there's that :P
P.S. Spell power cure+wormcult (or I forgot the other one, in case on of the trial healers already using wormcult) is still pretty much THE META if you want to do vet trials. bleargh.