There are a number of product characteristics that are not specified as codes so the type of MedicinalProductDefinition.characteristic needs to be a choice of, at least: CodeableConcept, String, Quantity, Integer.   There might be other choices that are applicable.

It also should be modelled as a name/value pair.  If I send a code of "Square", what is this characteristic?  It's probably the shape, but an implementer will need to imply that from the code sent.  That should really be modelled as characteristic.type = 'Shape' and characteristic.value = 'Square'.