Before I make suggestions, I would like to turn the question back to you and get some more information.
What do you think defines good user experience? Are there any existing distributions or environments that come close to what •you* think represents good user experience?
Do you want to contribute financial resources, marketing resources, development resources or ...?
I would try to think outside of the GNOME/KDE boxes if you can, but it is perfectly acceptable to think either of those are good representatives.
Are there any existing solution out there that you think come close or any blue sky projects that really excite you?
Rather than just say "You should support Project X", I would like to know more about your thoughts and opinions.
I think the ship has sailed long time ago. Today, when users hear Linux on desktop, it means you have to use the console to have things done, which isn't a bad thing but makes software makes expect Linux users to sort out issues when on Mac/Windows if you fail to deliver working software it means your product is not existing in main distribution channel.
Take most common software package, Firefox, here is how to install it on Ubuntu: https://support.mozilla.org/en-US/kb/install-firefox-linux.
You have to install a flatpack first. As a user I want to go the the software website, click install and that's it. To make Linux usable on a desktop, it has to appeal. Currently the appeal is it's free and some of the distros don't have a spyware/ads integrated, which is awesome.
I can't see any ways of Linux and its 600 distros becoming flagship user experience, freeBSD on the other hand might have a chance.
The biggest complaint is in the area of applications.
Especially from long time users of applications that don't run on GNU/Linux, who whine until they have a pixel-for-pixel, event-for-event exact equivalent.
"You can't be serious about about GNU Replacement Program; why it doesn't even have anything like the Floozney plugin. How can anyone use such a piece of garbage. And the UI looks like it's from about 2013."
Also, be selfish. When you're complaining about something, and the devs say "My grandmother can use it", don't just accept that their grandmother's user experience is more important than your own!