What about some crazy idea like a lottery/contest.

Premium Theme and Plugin developers could donate a theme or plugin to give away. But instead of giving it away, you can sell "tickets". Each ticket would be $2.00 and a packet of 5 tickets would be $7.50 (or something). Then at the end of each month, you'd award the premium plugin or theme to a randomly chosen ticket number. It would all be set up so that it would provide you an additional monthly stream of revenue, but it would also add value to the user's who are buying the ticket. It gives you a chance to win a nice theme or plugin for low cost. Now I'm not sure how legal it would be (in all US states and around the world), but it would be a simple way of adding some income, without selling advertising or something like that.