I know they're currently busy with other projects at the moment, but do you think we'll ever see them make a remaster of both Thief 1 and 2 with the KEX engine with new lighting effects?

I don’t think that replacing the Dark Engine with something else will ensure full compatibility with the thousands of existing fan missions. Even newdark sometimes has some issues with older missions. On the other hand, who knows, System Shock 2 EE seems to have support for fan missions and it's also based on the Dark Engine.