This got me good since I was thinking about getting the paid version of J.A.C.K.
and nobody uses the out of date free Jack, I hope.
I've been using the free version for ages now. What features does the paid version have that make the free that obsolete?

Speaking of ideas for a new modern map editor. I'd say GTK or Qt would be the way to go, maybe even Dear ImGUI. Also something that I thought would be cool is having a lighting preview option like Hamer++ has.

EDIT: One thing just came to my mind, a good portion of Linux users run gtk themes which may break the look of your editor to them. So be careful if picking that one.