Currently ForPartsOf accepts a single template parameter (a class), which requires the mocked method to be virtual. Making methods of a class virtual allows derived classes to override which may change the behavior of the class.

TImplementation Substitute.For<TImplementation>(...)

https://nsubstitute.github.io/help/partial-subs/

Suggestion is to to have an overload similar to

TInterface Substitute.For<TInterface, TImplementation>(...)

Any methods which are not mocked forwarded to the implementation class.