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I thought this had the potential to be a useful idea.
One of the issues I've seen is that it's sometimes difficult to determine the best place to as a question on SE.
To relieve posters and mods of some of this hassle, I thought it might be interesting to use tag collections to attempt to solve this.
When adding tags, if the user adds a tag that it much more popular on another site, recommend that the other site might be a better place to get the question answered.
For example, I recently couldn't decide if a Crystal Reports-related question (I know, please pity me) belonged on SuperUser or on StackOverflow itself. A quick tag comparison showed me that 10 questions were tagged on SU vs. 3500+ on StackOverflow, which helped me solve my problem.
If there was a way for SE sites to intelligently distinguish tag popularity at the time of question writing, it might help remove that question from the process.
Of course, care would have to be taken to ensure that a "tipping point" doesn't exist where a certain type of tag then will always be funneled to a certain type. I think this could be mitigated in the way the suggestion is presented (i.e. limiting how forceful the suggestion is in tone).
Just a thought I wanted to pass along,