In my imagined hyper-standardized Linux landscape, there'd be a standard for e.g. the semantics of interpreting metrics in themes, and a shared superset of required icons for a theme to have; such that you could create one theme once, and have it "just work", resulting in pixel-identical widgets, under GTK, QT, Wx, etc.
There'd also be a standard for e.g. IPC-based interoperation with file picker dialogs, such that if you were running GNOME as your DE, QT apps would (tell the DE to) pop the (GTK!) file picker to open a file, which would then signal back to the app through some standard message format for describing picked files (maybe with the file-handles themselves passed over a Unix domain socket, if you want something like macOS's sandbox-piercing file-open-intent tokens.)
There'd be a single "accessibility DOM" standard, so that a GTK screen-reader could read the text out of QT apps.
There'd be a single shell namespace standard, with one common set of abstract data types (with multiple implementations), so that opening some GVFS virtual folder in a KDE app would actually work — probably through D-Bus integration between the KDE app and some libgvfs host runner agent.
And so forth.
The ecosystems of each DE would remain distinct in development, with apps designed to fit well together... but the feeling of DEs being walled gardens would be gone, because the various standards would force each app to be a "chameleon" to whatever DE it's running under.
There are a few tiny attempts at this under FreeDesktop. xdg-open(1) is a good start. But we could be doing so much more.