Does having AST cancel the possibility to have Time Mage?

[Valic]

Maybe, but lore wise time is part of AST.

Though the removal of elemental stuff for WHM kinda hints at perhaps a Geomancer coming along.

I still am having trouble imagining how Geomancer would go... At least, how would it be presented as a unique healer ya know?

Not anymore, at least. From 3.0-4.0 time was a big feature in ASTs kit. Extending buffs, timed heals etc.

That's more or less gone in 5.0 and replaced with more generic heals, the only timed spell left is Earthly Star (which doesn't make much sense anymore now that you can't manipulate it's time but that's SE).

That's what I think too.