One small worrying issue I have with Dawntrail (Ending Spoilers)

[Voidmage]

This said, neither of those seem to be core functionalities of the device (key) since most likely it was created before sundering, although if it was it seems rather small for Ascian device.

This is also my opinion. DT, as bad as Wuk Lamat was, had actually a solid story ground with the other shard that was lost to lightning and Azem's hourglass (I call it this now).
Did I get it right that Sphenes world was actually rejoined in the past and only the small bubble and living memory remained?

It can be used for the future because we don't know what the actual function is and one thing they did right is to make it so it can be linked to Azem without going so much into it to give them any deeper character to damage headcanons of the players.

For me personally with it's shape it seems more like a device linked to time then dimensional travel and that the later thing is more a simple form of it's use.

I don't want them to ignore DT for the future but to build on the best parts of it. Ignore Wuk Lamat and Tulliyolal and embrace the deeper lore. Let us go adventuring while maintaining the curiosity of stuff like this key. Make it less about one single character and allready perfect harmony cultures but about us exploring the world being part in the change in it.

You know now that I think about it this is probably the biggest fail of the story.
It was so badly executed that the only topic is Wuk Lamat. Even here she overshadows all. xD
In other expansions you would see lore threads about the key and the path forward but not this time.
That is for me the biggest telling of how the MSQ failed.