Well, it's kind of obvious that Slayer aka Lord Luc is Isam, Lan's cousin. Lan's Aunt betrayed Malkier then went off into the blight with her son Isam, and was presumed dead. However, both Nyneave and Perrin think that Slayer in the dread world looks like Lan, he hints at claim to a throne in the borderlands, and the trollocs and Padin Fain call him Isam. Fairly convincing. However, There are three big questions about him that remain (I guess this isn't much of a theory)

1. What happened in the blight, how did he get control of Trollocs?

2. Why does he look different in The Dream World than real life? Perrin thinks he looks like Rand if anything... I think one of Rand's blood from Andor ran off into the blight, not sure though, a meld of the two maybe?

3. Why the hell does he run into the tower of Genji when perrin chases him in the Dread World?

That leads me to a different theory. Birgitte tells Perrin the only way to beat Snakes and Foxes is to break the rules. Why do they need to be beaten? Moiraine is stuck in there. Thom is a bard... no insturments is one of the rules, Moiraine knows who she's gonna marry, Thom likes her, etc etc etc He'll stroll in somehow strumming his harp to rescue Moiraine.