Global recipients and tag-based Mailing Lists
Once you start having 5+ mailing lists with recipients belonging to one or more of these, it becomes somewhat cumbersome to manage adding/removing recipients from the lists.
If the recipients would be global and mailing lists simply tags that could be attached to recipients, managing recipients would be easier. Then you could just call for example DELETE /v3/recipients/<email-address> to completely remove a recipient from all mailing lists. Similarly you could add a recipient to multiple mailing lists using a single POST.
I realize this is a major change, but I think it would simplify the platform in the long run. Also this seems to be the approach Mailchimp is using.