I'm coming late to the party, but wanted to point out this umbrella JIRA: https://issues.apache.org/jira/browse/SOLR-6082.
The current admin UI carries a bunch of historical design from a very long time ago. It seems to me that SolrCloud admin could be made much more user-friendly if we moved all the SolrCloud stuff to it's own page or something. Having a "select core" dropdown as a menu choice on a page that displays the state of your SolrCloud is...wrong.
Ideally, I'd like to see all the nodes in my cluster (whether they hosted collections or not). I'd like to ctrl-click on some number of them and be able to create a collection on the selected nodes. I'd like to be able to ctrl-click on a node and add a replica on that node to a collection. I'd like to.... you get the idea. All without having to drop into the shell prompt and use command-line scripts. Or type in a collections API call. We have the infrastructure in place, much of this would be a UI for the Collections API.
Note, I am not advocating we delay these issues waiting for some grand new design. Mostly I'm wondering if there's enough interest in this kind of thing to start designing a "SolrCloud admin interface". We can use SOLR-6082 as a basis for the discussion if so.