Xcode 8: Loading a plug-in failed
I had the exact thing happen to me except on High Sierra. I had deleted the old version folders of Python in /System/Library/Frameworks/Python.framework/Versions/, which was a mistake seeing that these are the Apple installed Python files. After trying to launch Xcode, Xcode could no longer access the Python files it needed. Unfortunately I had deleted them and emptied the trash, so the only way I could restore those files was by reinstalling High Sierra. 
So if you run into this plugin error and you've messed with Python files, you need to recover those files either by taking them back out of the trash or by reinstalling your operating system (reinstalling doesn't erase the data on your computer, but it will add missing files, such as the Python ones I deleted).
Hope that helps someone in a similar situation.
when launching Xcode beta 8 on a macOS Sierra beta I'm getting this error:

Loading a plug-in failed.
  The plug-in or one of its prerequisite plug-ins may be missing or damaged and may need to be reinstalled.

After searching, it seems that the issue is related with python and the new security measures that Apple introduced after XCode Ghost.
I couldn't find a solution, anybody can help?
EDIT
By looking at the Xcode logs, I noticed that it has NOTHING (apparently) to do with Python.
I see a whole bunch of
*Requested but did not find extension point with identifier Xcode.**
errors
I have to say that I also have Xcode 7 installed on my machine.