Either in place of the existing API URL / API Key or alongside it, we'll want to look at updating the WordPress site connection to the associated Mailchimp account via OAuth: https://mailchimp.com/developer/marketing/guides/access-user-data-oauth-2/. The first assessment here should likely be to determine the level of effort to implement this and then discuss with Mailchimp as to the feasibility to include this within the 1.6.0 release or perhaps push to a 2.0.0 release (as well as whether this replaces or augments the API URL/Key implementation).