It's an interesting idea, but the API surface is huge, so would take a great deal of development time. Then I'm doubtful how much third-party use there will be anyway - will someone really work for months to add major new features? Also many of those things in your list would basically have to be built-in features anyway, for architectural reasons, so an editor API probably wouldn't be suitable for building them. (Some of them are in our long-term plans anyway.)
We're currently working on drawing plugins, which just for that involves a large API surface, then we'll probably look in to custom importers so tools like Spriter/Spine can handle importing themselves. Beyond that I think we will focus on adding specific features that are definitely needed by a third-party addon, so we'll add things as needed from there.