I have tried to use the reusable blocks for form submission; however, I don’t want the form to submit with the same subject each time or maybe I want a different email.

Using reusable blocks, I have to use that block, and then break it down into basic blocks to avoid having the form send using the same info EVERYWHERE on my site (which kills the power of reusable blocks creating a consistent look). It would be great if I could mark parts of reusable block as globally changeable / locked, and the other fields as changeable for that “particular usage” (post, page, etc.). You can still keep the main block template to populate a default setting in the particular (i.e. instantaneous) usage.