Yes, you can go straight to Bitterblack Isle towards the start of the game, I think. I really wouldn't recommend that to new players at all. For first time players, the progression should be main campaign-->Everfall-->Bitterblack Isle
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The content on BBI is end-game. The multiple tiers of weapons and armor you'll get from BBI are better than sets from the vanilla game, including the rarer stuff from Ur-Dragon and Everfall. It is meant to be played last.
None of this is to say the campaign and main story is bad, but the game mechanics and combat aren't really pushed to their limits until Everfall and the new Dark Arisen content.