Software licenses

[william humphrey]

I was wondering. We can release a compiled version of a program written with
RunRev and Valentina database as "freeware" with the "share alike license"
but what about opening up the code under a license that allows improvements
and asks for those improvements back?
All this would take is distributing an unlocked RunRev stack and a Valentina
database and then telling the downloaders they have to go buy their copy of
RunRev and their own copy of Valentina to use it?

I guess this would be exactly like someone doing a similar thing with a C+
program?

What is the best license to use in this case?

The GNU public license is the one I like where anyone can make changes in
the software as long as they make those changes freely available.

Perhaps I should release a free compiled version for each platform and then
also the source code with the GNU license and as well links to RunRev and
Valentina so potential software-improvers can easily come up to speed?

I haven't seen any RunRev projects released like this but in my industry the
software costs millions of dollars (shipping - manifest generation stuff )
and I'd like to see what happens when a similar product is released for
free.