At present, StackExchange sidebars feature ads for questions at other SE sites. At present, these questions are chosen automatically - I believe it's high-voted questions with high-voted answers, like the newsletters.
However, sometimes there are other questions which may not be as active, but which might be a good choice to promote across the network. What's popular for a wide audience might be different than what's popular among a group of devoted enthusiasts; similarly, some questions may be opportunities for a beta site to branch out and stretch beyond its present scope.
For example, this idea occurred to me since Writers.SE got a pair of excellent questions in the tech-writing field, which is on-topic for us but hasn't gotten a lot of activity. I think these questions would be great to promote, because a lot of people outside our current user base might be interested in them. However, I have no way to mark these questions for any kind of promotion.
Hence I suggest there be some way that a beta community be able to mark certain questions for promotion, beyond the automatic selection.