As I commented on @SSumner's answer, even in canons where I think there is potential for strong overlap (I used the example of Lord of the Rings where I believe such potential really exists), many fans are just fans for the work's own sake, and not really interested in the deeper background and it's connections to our own real-world mythologies.
To be specific about the Lord of the Rings example, we have quite a heavy proportion of questions asked by watchers of Peter Jackson's movies who are looking for further explanations of events in the movies themselves but which may not be totally clear from the movies.
By way of comparison, we currently have 981 Lord of the Rings questions, but only 119 Silmarillion questions (this is going by tag counts and I'm ignoring mistaggings here). Of Tolkien fans, it seems to me that those who would be more inclined to follow Mythology are also more likely to be those with a deeper interest in the Silmarillion and related works, and we have relatively fewer of those.
While I am interested in this proposal I am not committed. The areas of mythology I am most interested (but by no means even a semi-expert) in - Celtic (particularly Irish) and Ancient Near Eastern - are under-represented in the example questions (there's the expected heavy skew towards Norse and Greek, with a good sampling of Egyptian, which just makes me cold), and while I could redress that by committing and participating, it's also the case that my interest is more in terms of what those mythologies reveal about, and how they link to, our own unrecorded prehistory rather than the mythologies themselves.
In short: I like mythology for what it tells us about unrecorded prehistory, but my interest is in the unrecorded prehistory part substantially more so than in the mythology part.
Consequently I feel that the site would be of quite marginal use to me. I will definitely be joining and checking-in semi-regularly, however.