So, here I am with a sack full of bears' hearts or some other gory trophy for some lazy peon, and I'm heading back into town.
How cool would it be if on the route home I got a whisper from some shady character lurking in an ally way, and he says "hey, don't be returning them thar parts to that lazy peon for a pitiful reward -- give 'em to me and I'll make it worth your while."
The storytelling opportunities would be interesting.
Currently, quest lines are strictly linear, with only a few exceptions. While there are many cases where the completion of one quest opens up three or more dependent quests, all which must be completed before the next link the the chain unlocks, I can't think of any substantial quest line which forces a choice of a branch.
Another dynamic is possible too for those quests which are started by an item routinely dropped by a mob - you might be given multiple choices as to who you could return it to, with each quest giver starting a different branch.
For example, Daniel Whitcomb at WowInsider presented this scenario:
Say you intercept a messenger from the Venture Company speaking about a stealth clear cutting operation in Ashenvale. Do you send word to Astranaar and help them ambush the logging crew, saving the trees and becoming a hero of the Night Elves? Do you negotiate with the Venture Company to keep quiet about the operation for a cut of the profits? Or do you offer to help them slip past the Night Elves and into Ashenvale, setting up the possibility of further business dealings with them on down the road?Now that would be interesting.
One caution though: you can't afford to design dynamic quest lines like this as a rare exception. Since the players would be trained by repeated exposure to linear quest lines, they wouldn't even stop to consider if they could hand it in to different quest givers, instead handing it in to the the first obvious one they stumble across. Stupid mistakes will occur if branching questlines are rarely encountered.