Well, I've been using Umbrello in the past on my Linux machine which admittedly is far more complex than I personally need. But I've found it invaluable in planning some projects.
Calligra Flow is another, but I believe it's been abandoned.
Regrettedly I lack the skill to port software or write such engineering wonders from scratch. My stuff is simple (yet I still need flowcharts- lol).
Writing such an application is much easier with a good vector graphics engine in place. Fortunately we now have ThorVG (https://www.thorvg.org/) in beta as a shared library so I think this would definitely be feasible for someone interested to take up that job.
A more complete vector graphics application would also be cool. I don't think I have the time for this myself though.
Then I guess a vector graphics application would have more interest and thus a higher priority. It would be very usable, the only thing I'd personally miss is a small pop-up note explaining in more detail what each element on the chart does when it's clicked. Maybe that can be done through some kind scripting anyway?
Pagestream sparks my interest, and is something I'd like to support sometime. But seems very complex and with a lot of windows to manage, and too expensive for me at the moment.
It's very kind of you to volunteer to start a project like this, but it might be good to gauge the overall interest in the community before committing to such a task. I'm sure there's a bunch of other things far more important to get done.
Not trying to be rude or anything, just making sure your time and energy is well spent.
