A book expected to be the biggest of the three's worth of time.

It just doesn't to fit, somehow.

TOM will already bring together Mat, Perrin, Rand and Egwene. This is already a major, major step forward for the Light.

If you had the alliance with the Seanchan into the mix, Jordan would have been completely left without an "internal conflict" within the Light.

As things stand, Perrin has made an alliance, but also defied Tuon's right to conquer. Mat has married her, but expects to face her on a battlefield. Rand has given up on her, and to Egwene she is a major enemy.

That would make for a very anticlimactic third act, if everything has been smoothed out among the Light.

Jordan has spent most of the series building up the Seanchan vs. Westlands conflict in the background, and it's gained more and more importance in the later books.

Personally, I really expect the resolution of this conflict will be a major part of AMOL.

At best, we won't get a resolution of this in TOM, we will get a further escalation of the conflict, leaving the Light facing Tarmon Gai'don and major open war with the Seanchan.

It makes the most sense to me, when you look how Jordan built it all up. Rand's failure with Tuon has been a major part of TGS, with foreshadowing in the book that now the future is in jeopardy, and Rand will have two face a two-front war. It's better and it's worse than he thinks: the Seanchan have already opened war against him (they think) by attacking Tar Valon. So the Light is already at war with them, but this is a major point that could make Egwene and Rand understand they need one another, badly. The other major point introduced by Jordan (and this too has been long set up), is the theft of Travelling. He's obviously building up to some climax, making the Seanchan suddenly far more poweful, more threatening. It's a clear sign the war will happen, to build this up in act 1... and that sort of things don't get resolved in act 2, which typically deepened the conflict (it ends with the first battle of TG...) It would be quite anticlimactic to make nothing of this, to smoothly end it with an alliance with both Rand and Egwene, when TG has not even started. The Seanchan are the last big thorn in the foot of the Light, that jeopardize the common fight against the Shadow. It won't get resolved easily.

Beside, there's too much that needs still to be set up before an alliance can happen. Mat is certainly to become involved, and Mat won't be reunited with Rand and Egwene before the reunion that marks the climax of the book. Mat won't have enough book time left to get involved in this.

Then, Egwene's dream isn't foreshadowing a smooth alliance. It's rather pointing to a desperate one. She is there, struggling to get on top of a sheer cliff, and she can't see the top or reach it. She cannot see beyond the clouds (she thus can't see the sun, the Light). The Seanchan woman tells her they must help on another to reach the top. The symbolism is rather obvious: the light is figuratively obscured, and neither Egwene nor the Seanchan can reach it without one another. This seems to point toward a resolution in a fairly dark hour of the LB, a desperate resolution when the two bitter enemies must understand they must ally, because they can no longer reach the light on their own. There is more than Tuon to convince: Egwene has even refused to see the end of this dream. She rejected her vision, and was thrown out of it. She needs to have new opportunities, new, more urgent dreams. At the moment, Egwene would reject out of hand any idea of a truce with the Seanchan. She hates them as much as the Seanchan hate free channellers. At this point, Rand is not in mood to seek any truce either. He's resolved that he won't have no choice but to fight the LB with the Seanchan at his back, hostile.

I'm not saying we won't see the Seanchan at all in TOM, but if we see them they won't quite be allies. They will be an even fiercer antagonist. Rand won't be happy one bit to learn that the Seanchan can strike so far to the North, and he will be catastrophied to learn they most likely stole Travelling. It's probably not another attempt at a truce he'll think of, it's more likely it will dawn on him that want it or not, he needs to cripple them enough they'll need to spend much time to recuperate, and won't be able to bother them any more during TG. My feeling is that Rand will ask Mat to design and lead this campaign. He'll learn then that Mat is a Seanchan Prince married to Tuon, and he'll tell Mat he must choose a side: his wife, or the Light. And Mat will face Tuon on the battlefield. Mat isn't going to bring much hope to Rand/Egwene. He knows Tuon can't be swayed. He's tried, and he's failed.

There's also the bit about Perrin promising to defend Ghealdan... where he'll be before the end of the book. And to had to the mix, he's gonna be involved with a man certain to hate him and his with a passion: Asunawa. Asunawa stands with the Seanchan. Galad stands against them. Berelain stands against them too, and Faile will as well. And Morgase will as well.

That conflict is really not over yet... if it plays a part in TOM, it will only deepen.