It would be great to have a "Query" parameter type for the following purpose:
Query for certain entities
Using entity query mechanics, providing configurable input for each field
Optionally restricted by bundle, similar to the Fields parameter type
Optionally limit the maximum number of items to receive
Bonus: Support count queries - probably as its own parameter type, because the return value changes from a list of entities to a number.
Bonus: Support aggregate queries - probably as its own parameter type, because the return value changes from a list of entities to a map of aggregated values.