You can always make two versions, one with TinyMCE and one without. The one without can be any license you want, while the one with TinyMCE would be provided under GPL.
One way many programs do in that situation is to redesign the program with plugins in mind. Those that wish to use a strict set of MIT parts can choose to do so, and those who do not care about licenses can use all parts. A alternative way some program do is to use compile flags, like ffmpeg.
It depend of course how heavily tied to TinyMCE your work is, and if multiple versions are worth it. Forking is always a possibility, but unless there is zero threat vectors I would not go that route.