A standard JSON implementation to rule them all:
There's actually software like Affinity Designer that supports exporting to Spine JSON
Please make Spriter receive JSON so Spriter can receive sprites directly from graphic applications and then you (Brashmonkey) talk to Affinity so they add an option inside Affinity Designer so Spriter can be part of the workflow too.
Please also add first party (developed and supported by you instead of third parties) runtimes (instead of first party converters) for Unity and most used.
If possible add real time editing directly from Spriter into Unity like Toon Boom does.