@MEB:
Thanks for your insights. There are very valid points that you pointed out. I had to rethink several of my opinions and came to one conclusion: I see one point different from a software developers view:
- Opening your software (or file format) will not broaden the scoop (in the meaning of "bloated") of Affinity Designer/Photo/etc. The software will stay the same. Not bloated or anything, if done correctly, now a days a "standard" (see REST API).
But as you mentioned, it would be the best idea to have a specific software for this purpose ... but actually what is the purpose? Purpose for a web developer? For a pre print engineer? The secretary that wants to print personalized invites based on Affinity Designer layout? In my opinion, this will not make anything better, because the scope to cover for this software is very broaden. So the best way is to have an API and a easy, if not already picked up language, like JS, Python etc. to fulfill your end result.
With an API it will be possible to write plugins for specific needs. Actually, a plugin IS what you named "yet another Affinity application".
Greets and thanks for all the inputs!