> design a universal app engine

You've reminded me of the XKCD comic about standards: https://xkcd.com/927/

Do you really want a universal app engine? If you don't have a good reason for ignoring platform guidelines (as many games do), then don't. The best applications on any platform are the ones that embrace the platform's conventions and quirks.

I get why businesses will settle for mediocre, but for personal projects why would you? Pick the platform you use and make the best application you can. If you must have cross-platform support, then decouple your UI and pick the right language and libraries for each platform (SwiftUI on Mac, GTK for Linux, etc...).

Please, for the love of all that is holy, not GTK.