This is something I’ve really wanted for awhile, but the plugins so good I’ve been fine with the repeater as a workaround.
I think for a first iteration I’d start small and abstract. For example, similarly to gravityforms you could include an input field for element class that applies to the parent element. That way anyone could easily add their own layout styles using bootstrap, foundation, or whatever home brew method they use. Especially since these classes would apply to the frontend form as well.
