Teleportation is broken, period. It was broken in WoM, to the extent that I'm shocked this spell even made it in.
The player uses it better than the AI, it completely overrides normal movement once you have a significant mana income, and it makes distances nearly irrelevant (ignoring abuses with outpost borders entirely)
At most, I would say a high level research that allows an expensive pair of gates between two cities to be constructed, allowing one unit at a time to pass through per turn or something along those lines.
Allowing stacks of doom unlimited mobility is a bad, bad, bad idea.
Giving the spell a significant movement bonus instead would be powerful but not hilariously broken bonus.