Sorry, but no. One serious problem with this is that communities around different disciplines have different norms for talking to each other. Having distinct sites allows the rules to be customized to each. For example, artists might want to talk about the “feel” of something, which would be way off topic on an engineering site. On the other hand, telling someone in an engineering context to read the manual, or even using the common short form “RTFM”, isn’t really unusual, whereas it might be taken as a serious insult in an artistic context. Trying to decide what rules apply from tags doesn’t work because it’s not about the content of the post, but the intended audience.
Another problem is that common words can have very different meanings depending on the community. Take “shield” for example. It means one thing to historians, something completely different to electrical engineers, and something completely different again to users of the Arduino microcontroller development boards. However, each community might want a “shield” tag.