What is missing?

It would be useful to add support for additional server-side filters when listing Cinder volume types in gophercloud in order to avoid redundant client-side filtering.

Using the Cinder API directly (via curl), it is possible to filter volume types by name. This suggests that name-based filtering is already supported internally by Cinder, but currently not exposed through gophercloud.

Additionally, Cinder allows filtering volume types by extra specs. For example, volume types can be filtered by storage_protocol using the following request:
GET /types?extra_specs={'storage_protocol': 'nfs'}