Using namespaces and types from known Unity PM packages should show a quick fix to install this package

Now that Unity is moving more towards packages; it is easy to try to reference a type (e.g. UnityEngine.UI.x) however since this has been moved to a package, you'd have to manually install the package first.
Rider could keep a list of known types/namespaces and use these to install and reference those packages for you. Referencing would obviously only work if you're already working in an assembly definition, if you aren't then Rider could suggest to create one or show the user an error.

Start by tracing how Rider handles unresolved Unity types and namespaces, then inspect the package and assembly-definition integration points. Define how known Unity package references should be detected and how installation or assembly-definition guidance is presented. Done means a reference to a package-provided type offers an appropriate quick fix or clear error when no assembly definition is available.

Written by the indexing model from the issue text.