Maevars assassin?
Sorry, a quick search revealed nothing. And while the logical presumption is "if we're not talking about it, then we've heard nothing about it", I might as well check.
The fallen paladins. You know, the guys the Radiant's send you to retrieve a cup from. At first, it seemed so similar to the Maevar quest, I was certain it would end up with a whole arc dedicated to it, the possible redemption of the deluded but not evil paladin guy, the slaughter of the totally evil paladin guy, and possibly an alternate path to Spellhold to those not inclined to deal with vamps or rogues. Instead you infiltrate the group... then they attack you and you kill them all. Really seems like an abruptly cut quest path.
Also, the female assassin at Maevars place. She has a unique personality, dialogue, and even her own voice acting. But nothing is ever done with her (that I know of)
?