I don't need the code for a UI system, I just need a design/graphics program where I can design the way the UI looks before I actually start building it.
I don't plan on using a premade GUI library like GTK since I want full control over the way everything looks and works and I want it to work simply and logically. I've worked on a UI system in the past, though it's not finished so I can only use it for very simplistic things for now. I'm not sure about making a "replacement" for those things though, I think generally speaking making a "do it all" system like that just ends up in a mess that's more complicated and slow than it needs to be because it has to account for every possible scenario. Who knows though, maybe it'll end up being something like that.
I plan to show settings that lets you scale the UI (and do other tweaks and changes) when you first run it.
In terms of design my #1 priority by far is clarity and ease of use, I'd rather make it ugly than make it hard to read and inconsistent and hide settings just for the sake of cleanness. Pic related is the latest UI mockup for the bigger program I mentioned, though there's a lot it doesn't show.