Nerfing the range will mean needing to redesign and replace many hidden treasure nooks, as well as the Gawin quest, which would become impossible to complete.
Other than the Gawin quest, I think anything you can access with teleport you can get to with a self-teleport skill. Slightly reducing the teleport range and making the gap to teleport Gawin a little smaller wouldn't be too much work I don't think. A simple nerf could be that teleport doesn't benefit from high range, which is often what really pushes it over the edge though that's a bit inconsistent if just that one spell can't work like that. Or just making it 2 AP would help. More teleport immunity enemies + self-teleports or other ways to counter teleports for enemies (like netherswap) as well. Plus at least an aeorothurge 2 requirement. I'm tactician mode will give enemies various tools to deal with teleport. I think several changes are in order.
I don't think the game is being balanced around teleport, but teleport does trivialize many situations. Nevermind the fact you get teleport for free on one character with the gloves
