The complexity comes with interaction in general. Writing plugins on their own is simple. And defining a good API for most common use cases is also not hard today, as we have enough experience now. But the hard part will be defining an independent Interface which will work everywhere on a satisfying level.

I mean that's basically the biggest selling-point of plugin-systems today. Not that you can write your own functions to do something, but how well you can integrate it into the interface and deliver your own interface to enhance the app. And another part would be safe interaction with other plugins, and necessary package-managment.