1) Editing paid invoices shouldn't be allowed after a payment has been made.
2) Quotes should have an accept or sign option, the quote list should reflect the status.
3) Quotes should also have templates similar to invoices
4) Dashboard widget reflecting outstanding payments, recent payment stats, etc. Could be handy.
I used to have other plugins that were sold directly, plus a couple in development which I may sell directly as well. When selling directly I have full control over the updates but through Codecanyon I do not have that benefit.
My preferred option would be to add support forever (helping people with the product, questions and so on) but actually limiting updates for a certain period, perhaps 1 year. That way I am paid for continually developing and improving the plugin but customer support is included always, which I think is important as a small business.