This is very far from my area of expertise, but I feel I should mention that Eclipse seems to be usable as a "framework" of sorts to make other apps. I daily use these two, wildly different, apps. Plus it's a code editor / IDE in its own right. It is my understanding that DBeaver is mostly built by one person, and the same is true with WireframeSketcher. Both are available for Win/Mac/Linux.
DBeaver (built using Eclipse)
https://dbeaver.io/
WireframeSketcher (built using Eclipse)
https://wireframesketcher.com/
(I get the sense that some people don't like Eclipse, I have nothing to say about that - I was just surprised one day when I realized two of my main apps were built on top of it. Nor do I have a clue if it's even possible/realistic to use it as a base for B4X IDEs.)