Hustle looks great. Before I can replace my existing opt-in plugin with Hustle I hope to see some additional features. Here's one that I personally need (and hopefully others do too), and another one that I think will help Hustle compete with the other big plugins.
1. MailerLite API integration. If you haven't used MailerLite, please try it out. It's amazing. I moved there from MailChimp. It's not as robust, but it's far easier to use and works great.
2. Content locking opt-in forms. Basically, an inline form that requires an email before the user can view the content on that page. Or even better, lock individual bits of text, images, buttons, etc. Any content, really. See this feature set in ThriveLeads, and also Monarch, among others.