Having responsive widgets is awesome, but I think all elements should be responsive without the use of widgets. It's a hassle having to make every little thing into a widget. Also for many stray elements on a page, having a widget definition that can be updated and reverted makes no sense.
I'm thinking a "Responsive" toggle that automatically adds a breakpoint selector for that cell's properties with all breakpoints active.
Of course, this feature would alleviate some of the hassle in a different way: http://support.antetype.com/support/discussions/topics/1000036769
But even if that request were implemented I think it's confusing to a new user that you can revert all of a cell's responsive properties with a single click. Having a widget definition for non-reused objects doesn't make sense.
Adding a "All properties individual" checkbox on the new widget sheet would be an alternate way of addressing that issue, though not intuitive to new users.
That's the biggest one for me. Other big items you might want to think about are a responsive grid system, and moving away from pixels and using points instead.
Other smaller things to add are proportional image scaling and filling, as well as the ability to reverse the flow order of the contents of a cell.
The "max cells per row" feature I had previously requested would be another great addition, as well as auto margins.