Well, if you had a website that cataloged and displayed the streams, much like Twitch.tv, then people can still find streams and subscribe to them in much the same way they do over at Twitch, while the site gets a cut.

Additionally, broadcasters can still do Paypal donation links, like they do over at Twitch.

Advertisements playing before videos, I'm not sure how it could be done, but it probably could be an overlay before the stream starts, or just a HTML5 video that loads first and must finish before the P2P stream begins.