One kind of ad you could run that wouldn't have trouble with cramming into a widget is popups.
Don't downvote me because you hate popups, so do I. It's a legitimate possibility that you might consider looking into. If it's a question of shutting down the service because of bleeding costs or running popups on 5% of pageviews, its an interesting question.
This is a great idea if you want to get rid of all your users. Nobody likes popups, and the people whose sites contain the widget would consider it a dick move and uninstall it immediately, I suspect.
Don't downvote me because you hate popups, so do I. It's a legitimate possibility that you might consider looking into. If it's a question of shutting down the service because of bleeding costs or running popups on 5% of pageviews, its an interesting question.