I agree with the difficulty with picking the next UX piece to build and as such I guess most of my "wants" are mostly for me. I spend most of the time in Jenkins fiddling around in the settings, plugins and credentials and such I'd love to see "/credentials", "/configure", "/configureSecurity", "/pluginManager" and maybe "/log" and "/load-statistics" under the Administration tab.
These would alone lower the need for the old UX for myself close to nil, as the pipelines are mostly automatically managed by our (Jenkinsfile based) integrations. I do see how the administration part will be really painful, as all the conventions used by plugins would be have to be "ported" somehow first I assume?
There's a lot of job level configuration that's moving into the Jenkinsfile with Declarative Pipeline , such as triggers, and you can expect that process to continue.
You're right on the plugin conventions having to be entirely ported. There's a whole new UI stack that those plugins have to adopt and we have to think carefully about how we design those extension points rather than blindly pulling them into the UI.