First, I'm really happy to see this plugin. I've been hammering you guys for a few months now to pick up your development and update your plugins to modern standards. I see Hustle as a big step on this path, as it looks to be a great framework for updating Pop-Up Pro and Slide-In.
I know many have already asked for more mail service integrations, and I know you're working on it. Nice to see you responsive to this and actively pursuing.
Along these lines, I would really like to see this plugin opened up to other uses -- including ones not specifically for newsletter signups. To start, I'd like to see an option for log-in/registration and for Gravity Forms.
I'd also like to see an option without a form -- making it a full replacement for Slide-In and Pop-Up, both of which are dated and in need of updating. Having a no-form option gives users a lot of options, which is always a good thing.
Another desired feature would be to direct users to a specific page after signup, either to give a better thank you, provide additional instructions, or to continue a form (such as allowing a new signee to pick from multiple additional newsletter lists).
Another desired feature would be prompts for social shares after a successful registration that allow for a customized message ("I just joined James Patterson's email list and got a free book!" sent to facebook, twitter, etc).
Looking forward to more updates for this plugin!