It would be great for popup forms to have the distinction between required and optional fields.
For example, our standard landing page forms might ask for the contact's state, but it's optional.
To create the equivalent form as a popup, we have to choose whether to keep that question and make it required (potentially over-asking and losing the contact) OR to leave out the question entirely (losing all possibility of getting that particular info from the contact through the form).
This would be a great feature for those who want to provide their leads an option to indicate their preferred communication method. For example, if they say they prefer email, there is no need to provide their phone number and visa versa.
I absolutely believe our pop up form would have a higher conversion rate if we could make the phone number optional. While we want it so we don't want to leave it off, we have had a few people say they don't like to leave phone numbers but will leave an email. We will take an email over no form at all. PLEASE add required and non-required fields on pop up forms.