Considering that we have Mindflayers, id say that the Underdark is almost a given.
Exploration wise, i think we have a safe bet that well get something like OS2: a big "open" map thats still, essentialy a string of "Quests" that are location based.
Baldurs Gate was no different tbh.
One thing i wish theyd do is to reuse locaitons. this might sound lazy, but i dont like playing "through" a map and never returning to older locations, it makes it feel stale and MMO like.
