Slide-in and pop-up forms are great for getting a reader's attention, but what if they aren't ready to share their email address?
Wouldn't it be nice if you could add a "Learn More" or "Download Now" CTA button to a pop-up? The CTA would send the reader to a landing page where they could dig deeper into your offer, instead of requiring them to provide their email sight unseen.
Hubspot's marketing blog has exactly this feature (see screenshot). Unfortuntely, it's custom-coded and not available on the HubSpot platform.
This should be a standard feature. It would easily become the most-used pop-up variation.