I was wondering if it would make sense to cross reference the tags we have for questions on different Stack Exchange sites. Some examples would be like the tags of "cosmology" and "astrophysics" which would be relevant to both astronomy and physics. Similarly there would be questions relating to astrophotography which might be relevant to the Photography community and its Stack Exchange website. (The examples relate to astronomy because that's the Stack Exchange site I have most experience with. I'm sure you could think of many more examples).
I can imagine that this would be an administrative hassle at some level, but maybe it's worth figuring out a way to unify across "different" "subjects". We could also have some aspects of user ratings independent of subject. Someone who contributes a lot in one field will definitely have a good handle on a related question in a different field. Such steps will increase interaction and bring in experts from neighbouring and related fields into a community. After all, knowledge is unified and we split it into different fields quite arbitrarily.