If someone with major resources announced a focused effort to enter the desktop OS space right now, I wouldn't put their odds in a good place. Why would an unfocused effort work?
I cringe at the idea of Linux itself containing a desktop, rather than distros adding one, so if that's the solution hopefully it keeps not being its year.
EDIT: clarification
Flatpack is an example of such an effort, but it needs to be more ambitious and preferably more “official”.
Flatpak isn't about APIs. It's about making easy-to-use application bundles.
The needed outcome is for freedesktop to have an application distrubution story - which is effectively the same as if flatpak had a larged application interoperability idea. The fact that this is two separate efforts highlights the problem - you need everything to be a coherent whole.
A combination of a larger and more modern desktop api compliance effort PLUS an app distribution effort like for example flatpak, would be perfect. E.g. "here is how you build an app for linux desktop v5". You include the right headers linux_desktop_sound.h, linux_desktop_dpi_notifications.h, linux_desktop_dpi_scaling_events.h (etc.). I know in the end an API on top just makes you have N+1 apis - but that's the reason this whole thing needs more dictator/ivory tower. It shouldnt' be be called "freedesktop-flatpack", or "somenewname" it should be called "linux desktop apps v1.0". This is probably the hardest part: getting the politics together. For desktop to succeed you need to have an ivory tower somewhere, in the middle of the bazaar.