Expand user contol over Exit Intent Popups
Give users more control over Exit Intent Popups on the dashboard. Let them turn on/off, adjust designs, A/B test, and allow different popups on different pages.
Re-populate forms when user returns to site
If a user abandons the site and later returns (either by themself or through a recovery campaign), automatically re-populate the form fields that they entered data into during their last visit. This will save time for user (and reduce friction leading to abandonment).
One-time abandoned product email blast
Ability to send a targeted email blast to visitors that abandoned a specific product during specific time period. For example, if you changed the price of a product you might want to email everyone that has recently abandoned that product about the price reduction.
Expand the weekly report to include a monthly option.
Instead of sending the campaign report weekly, include an option to send it monthly instead.
In Stock Notifications
Whenever a product is out of stock, having an email form field in place of the "add to cart" button that customers can enter their email address in to receive emails when the product is back in stock. There are other apps that do this, obviously, but I personally love cartstack with all of my heart, so I would prefer if they did it. I think one of cartstack's greatest strengths is the reporting dashboard, and I love having all of the analytics in one spot.
New split (a/b) test type for send timing
Currently, you can set up an A/B test for the subject line or the email content, but it would be nice to have a 3rd option for testing out the best timing of emails (eg: test sending a cart recovery email at 20 minutes after the abandon vs 1 hour after the abandon).
Spin Wheel gamification pop-up
Gamification and Spin Wheel pop ups for discount codes are way more effective than traditional "enter email for code" pop-ups. Optimonk is a good example. https://www.optimonk.com/ Could be exit pop or timed to show every visitor, etc.
Form analytics to discover drop-off points
Create a new checkout form analytics report that shows each field as a step in the checkout process and where users are most likely to abandon.
Get Emails Partnership/Integration
Get Emails automatically tracks visitors on your site and cross references multiple data points to get their email. It's completely within CAN-SPAMM act guidelines. But with my short experience with Cart Stack it seems you are open to creating/implementing cool and different features. If there could be a way to track visitors with an integration with Get Emails, it would be getting more email addresses with exactly what they browsed and did to send them a relevant message, that could be a huge win. CartStack customers get "free" access to Get Emails which further expands the effectiveness of campaigns.