I'd rather do it the other way round, as MP isn't an option for a lot of people, and it is often difficult to find good players. Just think what everybody is saying about BG2: the thing they like most is the replayability. So if you are going to put a lot of space to a good use, I'd say try to make each major quest have multiple possibilities/endings. I think most people would eappreciate that more than just a little bit of single-playing and then on to MP. It would mean that if you cannot or don't want to multiplay, the game hardly has anything to offer.
Just my 2c anyway.
I could try and think up some minor quests if you like, but could you perhaps email me a summary of what you've got so far (people, environmant, monsters etc.)
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Melusine, High Queen of Fluffies, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia