Bitterblack Isle wasn't perfect, but it was definitely better than the main game. Perhaps because it did away with the main game's weakness - i.e. a sterile open-world adventure with poor world-building and dull characters - in favor of what arguably sets it apart from its contemporaries: an exciting battle-system with difficult encounters. Heck, even the story bits are better for being peppered throughout the dungeon with narrated journal entries that slowly reveal what BBI is about.
Dark Arisen essentially turned a mediocre open-world RPG into a great dungeon crawler. BBI's only real fault is that it could use more level variety, both in layout and themes. Also, Death can suck it.
To me, an ideal DD2 would expand on the BBI concept, with multiplayer co-op to boot, more balanced classes and stat distribution between classes, and less spell-charging time for mage-type characters. Or, if they have the budget, time and talent for it, try going open-world again, sure, but don't screw it up this time.