I am now fantasizing about an ending where Demandred being Taim is not revealed to the character till the very end (even if the readers themselves figure it out). Imagine Rand's most potent creation for the Last Battle: the Asha'man, going so badly astray.
Imagine the true emotional weight the Black Tower storyline would have carried. Plagued from its very founding by the Shadow's agents, at last freed by a heroic few, with help from Aes Sedai.
That kind of thing would have not just made Demandred's storyline more interesting. It would have also meant that the Black Tower didn't end up as this hellish arc which showcased Brandon's "creation" (ie. Androl), but had the resolution of its arc happen backstage.
I too mourn the fact that RJ felt the need to change this story. Surprising the audience is fun. But some of the most satisfying events in the last three books were long foreseen. I wish RJ had anticipated that, and not changed this storyline.