It would actually be cool to off one of the two Forsaken dealing wiht Perrin or Mat (Moridin's two pets).

Cyndane, Moridin, Demandred makes a very likely "final trio" of villains, though perhaps Mesaana will be there too for a while. They are the three who wants to off Rand themselves. It's a good idea to keep around the one Forsaken who knows all there is to know about the Bore. This could be very useful down the line (for exposition etc. If anyone stands any chance of puzzling Rand's plan for the Bore when it comes - or at least speculate what it might be, that'd be Lanfear. That may be this expertise that has made the DO bring her back).

Moghedien's storyline has gone on long enough. The build up has been long enough, she's escaped from tight spots often enough. It's time to finally finish her off, I think. I think she's the one who will be involved with Mat in Caemlyn, so the timing is good. If anyone if insanely lucky enough to make Moghedien stumble out of the shadows long enough to get herself killed, it's Mat Cauthon. There's even something in KOD that might be seen as foreshadowing he will have to deal with an evil woman and not hesitate to kill her (Tuon and her blasting him for feeling bad for killing a treacherous woman). If Moghedien steps out of the shadows to kill Mat after all her other "spiderwebs" failed, not knowing about his medallion, she's in for a really bad surprise. It would be cool that one Forsaken gets killed with mundane weapons by a non-channeller too... Mat's medallion also protects Mat's dreams, which is very likely to frustrate Moghedien. Down the line, she may have no choice but to step out and deal with him herself, because everything else she throws at him, like DF, fails. She would hate that, but she might become so frustrated and angry at Mat (and Elayne) she might make such a mistake.

Mesaana's fate is more uncertain, I think. She may die in TOM and she may not. Unless Aran'gar returns as a foe near Egwene, that would remove all the Shadow at the WT right at the beginning of TG. An odd, undramatic choice. Unless Mesaana's fall is precipitated, the 4-5 expected Egwene chapters don't seem enough to deal with Mesaana yet, especially since their purpose is to bridge from the reunification of the WT to the big event that launches TG. Somehow, this makes me quite uncertain Mesaana won't be kept as a threat near Egwene for a while longer, to be dealt with during TG, not right at its beginning.