SE has some automated ads which take a recent question from another SE network site and present that question's title, tags, and the site title. It's a neat little way of showing what else is out there on the network. You know, these things:
I think it'd be helpful if the community ads were weighted a bit so Science sites tended to see questions from other Science sites, Tech sites see Tech ads etc. It wouldn't need to be completely insular; you could show some, but just less, ads from unrelated areas.
The SE network has grown large, and I think the ads need to be a bit smarter; users on Physics are very likely interested in the Biology and Chemistry sites, maybe even Cognitive Sciences. Prioritizing related sites seems like a big win, especially for the non-tech sites which tend to feel a bit more isolated from the network.
For reference, the categories SE currently categorizes sites under are: Technology, Culture, Recreation, Science, Life, Arts, Business, Professional
A possible problem I note is there are very few Business and Professional proposals. So maybe rather than setting a system where x% of ads must be in the same category as the site they'll be shown own, ads in the same category just get X% more weight than less related ads.
Ideally these ads would be hand-tailored of course; the Graphic Design and Cognitive Sciences sites are very related to the User Experience site, but they're all in different categories. But at that point you'd need more than a dumb weighted filter to tailor ads.