I asked before, but it must have gotten lost in the clutter.
Is it expected that this new system will be compatible with all classic datatypes, like the 3rd party warp datatypes, or will it require previous datatypes be updated to conform with it? I would assume so, and hope so, but it is never safe to assume.
I think that's already been answered as "yes".
I don't really see the point in rewriting datatypes.library. It doesn't really do very much - all the power is in the superclasses, and you can extend and write new ones of those as much as you like without touching datatypes.library. If you want to write a video.datatype you can, without datatypes.library knowing or caring what such a thing is. I wrote a structured art superclass for OS4 so IFF DR2D and SVG (and.. I think that's as far as I got) can be viewed by Multiview. It works, and programs can (in theory) load such objects retaining the original structure and write them as well (can't remember whether I ever implemented this stuff). datatypes.library doesn't care, it just looks at the IDs and passes everything down to the subclass, which it also doesn't know about as it was written years before it existed.
datatypes.library does have shortcomings, but none that actually prevent new formats from being supported, so I don't really see what this gains apart from potentially some slightly better APIs (which nobody will use because they won't work on the version everybody has).
(btw, the animation.datatype is horredous, if anything needs rewriting it's that thing)