I've had a couple clients ask how to get their form results to come FROM the person who submitted the form, currently it defaults to the main site email address. This would need to involve taking the value entered into a form field and passing it to the "from" address or "reply to" field of the email that is sent out.
Clients are used to getting an email from someone and pressing reply, if the email address isn't filled in they have to copy/paste and half the time they forget and send the email to themselves.
I know this would involve some checks for a valid email address and a way to assign a form field to use as the form "from" address. Could it cause email relay problems or potential hack attempts, spoofing? Not sure, but there must be a solution as you see this functionality on most forms. Granted on a form builder type of module it's more complex.
Just a possible enhancement to add to the rainy day list.