Currently, the only options for case field data types are String, Number, and Boolean.

it would be very beneficial to allow admins to assign a particular picklist value based on individual case expression criteria. My original use case for this is in Data Designer, but the option should be available wherever Case Expressions are used.
In Data Designer, you often have highly curated data sets, necessitating the creation of new attributes (fields) via transformations that don’t otherwise exist at an object level. Enabling picklists as a data type would allow you more flexibility in assigning an attribute to records in your curated data set, without requiring picklists as permanent attributes on your object level records.
Ideally, admins would be allowed to reference picklists in their designs, not in the design but in data management, as well as create a new picklist, when assigning the values.