But I actually don't think that Fain will yet play an important role. He already did that in the past as troublemaker, but also as the one who gave Rand the idea on how to Cleanse saidin. I would actually say that it would be a bit lame if Fain would again corrupt another group/character this later in the story (of course in the end without successfully killing Rand or destroying the Shadow by this), and also don't see whom Fain would yet try to corrupt.

The last act of the series will just see Fain's death, IMO. One can see a trend by RJ to quickly finish such plotlines by killing or disempower an evil character. I believe that Slayer and the DH pack will finish Fain in a memorable scene. There might be a twist, especially that it will remain uncertain if Mordeth's ghost really died with Fain's body, though.