Right now we only show a part of available timeline entity data in the new YARN UI. However, some data (especially library specific data) are not possible to be queried out by the web UI. It will be appealing for the UI to provide an "entity browser" for each YARN application. Actually, simply dumping out available timeline entities (with proper pagination, of course) would be pretty helpful for UI users.
On timeline side, we're not far away from this goal. Right now I believe the only thing missing is to list all available entity types within one application. The challenge here is that we're not storing this data for each application, but given this kind of call is relatively rare (compare to writes and updates) we can perform some scanning during the read time.