Nothing comes to mind anyway.. don't think there's really anything that is unavailable to you later on, though i'm not 100% sure of this as I usually do -everything- before setting sail to Brynnlaw.
And I mean -everything-.. once I get to Brynnlaw, i usually just go power-play and finish the game asap.
I usually go 50/50 on the quests and I haven't noticed any missing quests in chap 6. Case Hexxat is probably an isolated incident. Though I wouldn't bet anything considering the other EE-NPCs. I think they are all supposed to be 1st quest in chap 2-3 and the 2nd quest in chap 6.
Maybe the Skinner quest if you're playing evil. But if you've killed Adalon in the Underdark then you can most likely delay this quest until chapter 6 as well.
I think a few of the sidequests in Umar Hills need to be done before Chapter 4, at least the one with the gem in the chickens, I couldn't find them anywhere in Chapter 6 anyway.
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And I mean -everything-.. once I get to Brynnlaw, i usually just go power-play and finish the game asap.