I know the majority of this feature request list can be accomplished with custom html/css/js, but adding this functionality natively to the form builder would be nice 
Tablet Breakpoint
Add a third breakpoint in the designer for tablet devices.
Layer / Element Navigator
Add a list of elements / layers on the current form step. This would allow you to more easily select layers and rearrange them. Could be a third tab in the left Flow/Elements pane.
Button Groups - Ability to Set Number of Columns & Rotate into Columns
This feature would allow you set the number of columns used in a button group, without having to set percentages or fixed pixels which can cause styling issues when the form is responsive.
Superscript / Subscript Font Styling
Ability to format text as superscript or subscript.
Negative Character Spacing Font Styling
Ability to set a negative character spacing value on text elements.
Container Height Minimum
Remove the 15px minimum height requirement from containers.
Progress Bar Styling
Ability to set the thickness of the progress bar, text to add before / after the displayed percentage, and horizontal alignment of text.
Button Hover Shadow
Ability to add a shadow to a button on hover.
Button Description Text
Ability to add a second line of text inside a button or button group button.
For example:
BUTTON CHOICE
This is a description of the button
Button Group Radio / Checkbox
Ability to add a checkbox (multiple selections) or radio icon (single selection) to buttons of a button group. This can have a positive impact on form conversion rate.
Also like the button image feature, provide the ability to position the radio/checkbox as left align, right align, above text, or below text.
Image Upload Search Feature
Add the ability to search for existing images in the select image popup modal.
Image Hover Swap Image
Add the ability to specify a different image when a user hovers.
Image URL
Ability to display an image from an URL (without using a container element). And also support inserting dynamic field variables into the image url.
Dropdown Search
Add the ability for a user to keyword search through a long list of dropdown options (see thezebra.com for an example).
Input Placeholder Padding
Ability to set padding of placeholder text.
Form Live / Test Form Links
Add an option that will allow you to open and preview the form at the step currently being edited.