Is there a way to pull a list from mailchimp with all the "custom fields"?
Supermetrics should have all the available metrics/fields that are available within the MailChimp API. You can check the list from here: https://developer.mailchimp.com/documentation/mailchimp/reference/overview/ -and if you see something thats missing from Supermetrics, please let us know, send us a screenshot of it and we'll add it straight away. Thanks!
Unfortunately getting list-specific custom fields from merge_fields is not currently possible. Based on the API documentation, I'd say that a performance friendly implementation would also require quite a bit of effort too. I can add this to the development roadmap as a feature request, but unfortunately I can't say if or when it could be available for use... But still, thank you very much for the feedback!
If I could ask a quick follow up question, what exactly is it that makes merge tags any complicated than pulling any other piece of nested data, like name or country? The "name" field is already a parameter within merge fields, which itself falls under lists. Just seems like a GET request to pull in all available merge tags wouldn't be any more complicated than however you are currently pulling in other account-specific data like lists and campaigns.
But maybe there's more to it than I realize. It would just be really great to be able to get this data into my Google sheets.