I love the slide in plugin, but the MailChimp integration isn't really useful for my purposes. For me not a real problem, since it also processes shortcodes and I just added a gravityforms form in it which works well too.
But I have my list seriously segmented. I do have several free products (like e-books, trainings etc) and I want to track what people download, since that indicates their interest. Or, to make a long story short: I intensively use the mailchimp group on the background. Depending on the group they enter, certain automations will start.
So, I would need the possibility to 'set fields' after adding somebody to the list. Also, because I like 'personalized' mail, I need a first name.
As I said, all can be solved with GravityForms shortcodes, but not everybody uses that. It would add value to the plugin if you could optionally add a first name and optionally give the possibility to add somebody to a group (or fill some other field) when added via that form.