One feature I would like to see is the ability to send a PDF invoice when the user registers, and another when they have paid. The email function is OK but the formatting of the invoice is difficult with various email clients, so a PDF attachment would be a major advantage. Even if this was a paid add-on I would imagine a lot of people would be interested.
If anyone has a good clean html invoice (for the email boxes) I would be interested as I still haven't mastered one.
We are packaging up a new extension that will do this. Should have it done over the next 4 or 5 days. It is a paid add on and will allow you to invoice the client and additional line items to the invoice if you want.