Styling forms is hard. Wouldn't it be great if there was a Sass extension that would do just that, configurable to fit your needs? Here's a concept for Sass Forms (which needs a fancier name, obviously). Sass Forms comes with sensible defaults.
Because we only want to fight over Sass and the resulting CSS, we'll start with markup from SimpleForm, a gem wildly used in Rails projects (but you could just hand-code said markup as well). By doing this, Sass Forms will immediately work in thousands of web apps. By making the input wrappers, etc configurable, like SimpleForm does, it can be used in even more projects.
Later on, Sass Forms can also support Formtastic markup (or any other gem with predefined output). By tweaking just a few configuration variables, it's also great for forms with custom markup.
It should be decided whether we want to support custom form inputs, like custom selects, radio's and checkboxes (which can be a pain because of keyboard support, and requires JavaScript).
With Modernizr support enabled, we can check for touch devices (and detect other features), to create the best experience for as many devices as possible. Nice to have in the future though.
I don't see a chance to handle form-layout without a basic idea of the underlying structure. As CSS and HTML are tied together this is no real news.
On the other hand, if you only target classes without any reference to elements, different semantic approaches should be possible.
The only real broad approach to styling forms which is still in active development is the approach from YAML. You should take a look, the project is built upon Sass.
Maybe most of what you want is already solved.
Individual inputs or selects are mostly a work of JavaScript. And if you happen to create something with pure CSS you can end up hurting some browsers or accessibility. And besides that it is very indivdidual, so a collection of nice form solutions via Codepen would be a nice approach.