The problem is, that cutscenes require lots of data. Personally, I don't want to set them in WED or so.. because its too much unnecessary data binded in a level files. So I would have to load paths, timetables, scripts, ressources etc. during runtime - seperated from the WMP file. Plus, a realtime 3D editor with a preview function would be invaluable to artists.

TUSC failed for several reasons, especially caused by an "open end feature list". A cutscene editor would require indeed a specific interface for the game (in the meaning of accessing the editor and embedding cutscenes), but that could be simplified. All the rest is conventional software engineering with a fixed number of specific features. I don't know why you are kidding