Custom CSS section for each form

You could do this now, if you really wanted to put it inline, using the HTML field. You could add an HTML field to your form at the very top and insert your CSS in a style block within that HTML field.

An alternative if you want it separate from your theme CSS would be to create your own Gravity Forms specific stylesheet and then update your theme to output it along with it's CSS. Unless you don't want your theme outputting it at all.

Any reason why you want to keep it separate from your theme CSS? Typically the theme doesn't change very often and that is where most customizations reside so they don't get overridden by plugin updates.

One thing we will be adding sometime in the near future is the ability for Gravity Forms to identify the existence of a gravityforms.css file in your theme folder and automatically enqueue it when it enqueue it's CSS. What we may be able to do is include an option that could go in the wp-config.php that if present could tell Gravity Forms to look somewhere else for this custom CSS file, such as the wp-content folder, which could be outside of your theme.

A visual style editor is definitely something we have ideas for. We've avoided it because we strongly believe that the theme should drive the style of the site. But it's still something that we haven't ruled out.