Daring Fireball has ads in the RSS feed[1]. If RSS was more successful, we could imagine services, marketplaces for that model. Of course the same personalization you have on the web wouldn't be possible, but maybe in some ways it would be better, because you would know that everyone else sees the same ad, creating common ground[2].

[1]: https://daringfireball.net/feeds/sponsors/

[2]: https://meltingasphalt.com/ads-dont-work-that-way/