I'd argue multi-window would require a substantial redesign of the graphics layer, so much that the maintainers would be uninterested in supporting it, and consider it out of scope.

I can imagine IME support being integrated, but a giant pain, due to the wide variety of APIs across platforms, the fundamental disconnect in how text input is done in games vs. elsewhere, not to mention the "Linux wars": will you support ibus or fcitx?

So it requires someone to step up and do the work, and don't be fooled: it's a lot of work.