I think one of the problems with this is that it's not actually getting to the root reason for why it might make sense to regonalize the tags.
@NiallC referred me from a related answer I gave, where the question was originally about clarifying 120V/60hz vs 230V/50hz etc without being overly technical for users that may not understand voltage/frequency (considering how many people don't realize that "110", "115" and "120" are really all the same system).
It seems like generally local code questions are off topic (though being debated), and I'd suggest we keep that discussion out of these tags (there could be different tags for code questions, if we go there).
So if we scope this as changing the tags to better identify technical need (eg. voltages), then I'd like to see us add regions based on voltage/frequency, for the specific reason that knowing the voltage/electrical system in use is often critically important to answering electrical questions.
I also agree with the user experience aspect here: you start typing "electrical" and get the high-level regions to naturally choose from, and are implicitly telling us the voltage without having to know.
For plumbing or HVAC, I'm not sure there are such differences that can be identified regionally (maybe instead based on climate?). I can't even think of any other topics that may need this amount of categorization -- but if there is a need I'd suggest we deal with them based on the technical difference, rather than adding a region for the sake of adding a region.
*: That said, we COULD add a whole tag hierarchy for identifying the region, but getting people to use it will be tough, and it may not even be useful. At least as a separate set of tags, it can apply (or not apply) to any and all topics, and we can be as specific or broad as needed without having to affect
plumbing-* etc tags.