You should be able to A/B test a form in the same way you can A/B test emails or landing pages. When working on landing page optimization, the form is one of the biggest variables, yet it's clunky to test this in Marketo currently.
Forms 2.0 is a 100x easier way to edit forms, but it's still a pain to manage mutliple versions of a form (for example, a long and a short form) just for testing. You should be able to set up testing directly within the form asset and define tests such as:
Label text (eg "email" vs "work email" )
Form field order
I know it's technically possible to do this via a workaround with multiple form assets in a landing page test, but that's way too clunky, and creates this post-test detritus of assets you have to delete or file away later.