Hi Jan,
Re your third solution, that's very similar to what we have proposed in the API documentation with one key difference - just implement an interface if it doesn't exist:
namespace assignsubmission_example\privacy;
if (interface_exists('\core_privacy\local\request\userlist')) {
interface my_userlist extends \core_privacy\local\request\userlist{}
} else {
interface my_userlist {};
}
class provider implements my_userlist {
Andrew