The Field name should match the JSON tag: Name.

I used a different name because this API has proved confusing to users: it isn't checking for that resource type by name, but filtering the list by a regex. I'd change the API if I could, but although that horse has bolted it would be nice to avoid perpetuating the confusion here. Would something like NameRegEx be acceptable?

We can modify this a little to still use a default value but allowing a user to opt-out:

type ListOpts struct {
  NameRegex string `q:"name"`
  SupportStatus SupportStatus `q:"support_status"`
  WithDescription *bool `q:"with_description"`
}

func (opts ListOpts) ToResourceTypeListQuery() (string, error) {
  if opts.WithDescription == nil {
    iTrue := true
    opts.WithDescription = &iTrue
  }

  q, err := gophercloud.BuildQueryString(opts)
  return q.String(), err
}

This will cause with_description to be set to true by default, but allowing someone to opt-out and set it to false without having to override the method.

You know, if we have to include this option then with this output format I think it's fine to just have it default to false and not try to do anything clever.

Right, but I thought the idea was to have with_description set to true by default so the user receives the more detailed API response?

Originally I was looking through the API code and interpreted it to mean that the old format was there only to support older versions of the client. But on closer inspection, it means that not specifying an option is there only to support older versions. The current openstackclient CLI still defaults to false. And with this data structure as the output it seems less weird to leave the description blank if the caller doesn't request it. So I'm ok with defaulting to false.

Similar here:

type GenerateTemplateOpts struct {
  TemplateType *GeneratedTemplateType `q:"template_type"`
}

func (opts GenerateTemplateOpts) ToGenerateTemplateQuery() (string, error) {
  if opts.TemplateType == nil {
    templateType := TemplateTypeHOT
    opts.TemplateType = &templateType
  }

  q, err := gophercloud.BuildQueryString(opts)
  return q.String(), err
}

func GenerateTemplate(client *gophercloud.ServiceClient, resourceType string, opts GenerateTemplateOptsBuilder) (r TemplateResult) {
  url := generateTemplateURL(client, resourceType)
  if opts == nil {
    opts = GenerateTemplateOpts{}
  }
  query, err := opts.ToGenerateTemplateQuery()
  if err != nil {
    r.Err = err
    return
  }
  url += query
  _, r.Err = client.Get(url, &r.Body, nil)
  return
}

It'll just default to the empty string, so I don't think we even need to do pointers.

and also to have constants already defined to use in switch statements

Good point