Late to this party but...
I don't understand the concerns over game length. First, it's an expansion not a full game (and will likely be priced as such). Second, content is what matters.
ESPECIALLY in RPGs where game time is usually "padded" with monotonous back tracking, inventory management (in the case of games with poor inventory management systems combined with excessive loot), unwelcome grinding (not optional battles where one can build up ones character levels, gain cash or loot IF one chooses but constant dull, tedious unchallenging combat), long boring dialogue or tons of silly fetch quests.
I'd rather play a short game that is well balanced and has an interesting and well-paced story. But the main thing though is to keep the game challenging from beginning to end.
I think if a game is short but challenging (and entertaining, not frustratingly difficult due to poor design or mechanics) it makes it all the more worth playing through. If you feel like you've come close to breaking a sweat a few times it will make finishing it all the more sweeter and may even invite a second play through especially if it's deep enough to give you a different experience the second time.
I haven't played MoTB unfortunately (will soon though) but I would have enjoyed NWN2 OC a lot more if it had more difficult combat but less of it (would have helped greatly with the pacing).
Kotor 2's length felt just right but again a lot of that was spent in what I thought was very easy combat (and this on the "Difficult" setting) that sucked some of the enjoyment out of a game that had great dialogue and characters, a good story and some entertaining quests.
So from what I've read this new expansion will be nothing like MoTB which has gotten a lot of praise. Whatever you do, just make sure you keep the pacing and difficulty in mind. I want a 15 hour rollercoaster ride, where every battle counts and isn't there just to give me the necessary xp to fight the main nemesis, where the story twists and turns and leaves me wondering what will happen next, where the dialgoue crackles with life and energy, where the end credits bring a smile of satisfaction to my face not the feeling of "oh, finally finished now I can play XXXXX".
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