He was really useful to RJ in the series itself both has a foe/wildcard and as a thematic device to flesh out - literaly in this case, the evil of Shadar Logoth which was the path of darkness Rand nearly succumbed to and Masema succumbed to, where "the victory of the Light is all" and "victory would be as bad as defeat", but yeah, I think his role will be significant in AMOL, though this time it's the Shadow that will pay the most, I think. I'm not as willing to bet he will have a terribly significant role in the endgame itself, but my best guess is that he will clash with Shaidar Haran. We don't even know what would happen if SH striked at Fain with the TP - the result could perhaps destroy both. In any case, I can't think of anything that will be able to deal with SH except Fain and the evil of SL (also minimally present in Rand's side, however).

Mordeth is also tied to Mat, the taint of SL is what stole his memories, and this hasn't played out in the series, while Fain has clashed with Perrin and Rand. It doesn't mean Mat will too, Jordan has as often repeated patterns for only two out of three ta'veren (one for whom it's a major theme/event, one for which it's more minor) as he declined variations for all three.

A vaguely interesting tidbit that also links him to Mat, re: Mordeth and his knowledge that made him create a power to defeat the DO in Aridhol in the first place. Brandon or Maria has confirmed he got that from a visit to the Finns.

It's really hard to make guess about Fain/Mordeth, and I think this owes a lot to the fact RJ uses him as a wildcard and he always resurfaced where and how you didn't expect him. RJ did it again by showing him in the Blight in the prologue. It's a safe bet he's heading for Moridin or Shaidar Haran.

His powers are growing impressive. I suspect like you they're far more impressive than we can guess at now. There were allusions in Moiraine's speech to the "unnatural creatures" of Mordeth. Mordeth/the taint seems able to corrupt the pattern around him (in the land? In others? He seemed to draw the energy out of his followers) and tap in that corrupt energy and use it. Not channelling for sure, but something related to the OP (which is only logical if it's the driving energy of Creation..), if to a totally corrupted OP.