I've decided that, should the game actually last long enough to get the party through the Fane of Tiamat, that the Aspect of Tiamat isn't going to show up. I've started dropping hints that the known gods (specifically Celestia and Luna, but it applies to Tiamat as well) can't interfere directly with each other's people. I've also been laying down seeds that this entire invasion (and other things happening in the background, for that matter) is planned by a mysterious "other," which has ties to the Plane of Shadow — and that this other figure is what corrupted Luna more than a thousand years ago.

I'm imagining the look on Azurr Kul's face when, instead of a friendly dragon, Tirek shows up through the portal... and I'm imagining the look on my players' faces will be even more surprised.