If Tassadar is to return in LotV (which he probably will), I hope he doesn't do so physically, would be better if he was Zeratul's spiritual guide/mentor/advisor, imo.
Though, a sad/'bad' ending wouldn't be such a bad idea, or even a bittersweet ending. Then again, looking at the two SC2 endings so far, I have a feeling it'll be a happy one.
Also, thumbs up for less timed missions/timed achievements, it feels as if almost every level is timed in a way (especially in HotS). Personally, I like to take my time and explore everything, but if a time limit exists...eh. Also, more difficult-to-acquire achievements would be nice, especially for the Mastery-type ones. HotS' felt a bit too easy to get and the feeling of achieving something challenging wasn't as present as in WoL. :/