I think it would be helpful to be able to jump between the Routes*.yaml files and controller actions or classes.
Obviously there is a bit of flexibility in using the routes, but I would imagine

• click on defaults.@controller value to go to @package/…@subpackage/…@controllerController
• click on defaults.@action value to go to the corresponding action if available
• maybe have gutter icons in the controller class that show the routes that could be matched, e.g. with the resolved name (Package :: (Suffix :: )+ Name) and a link to the file

A more powerful interpretation of the pattern could be interesting, but probably a bigger task. Things like completion for routeParts or subRoutes based on the placeholders in the pattern...

Unfortunately resolving the sub-routes with merged defaults is a bit annoying, I imagine.
However after that I think the framework does do some shortcuts in resolving the class names; in general it might be interesting to have a utility to get paths, manifest etc. by package-key, but I think that's not strictly necessary here.