It would be extremely beneficial to allow read/write permission options per field.

I may not want Sales Users to be able to edit a certain field on the Contact or Opportunity.

Once a Contact is created, I may not want to allow someone to update certain info on that Contact simply by submitting an externally available form with that same email address.

I may not want certain API integrations, or other syncing options, to be able to update all fields during a sync.

I may not want certain CSV uploads to overwrite certain fields if there's already a value present.