Although this is clearly an unpopular position (and I'm sure I'll be voted down here as in my Is Cthulhu on or off topic?";) I think we can expand traffic by allowing modern mythologies to be discussed.
My reason is this: All mythologies are fictional. Do we merely restrict discussion to mythologies considered ancient? But then how ancient? Where is the cut off line? Gilgamesh and the Prose Edda are far more temporally distant than the Prose Edda and Lovecraft. In 10,000 years, will D'Aulaire be taken as canonical? His work will certainly be seen as quite ancient to a person of that time. Not to mention that D'Aulaire was my introduction to both Greek and Norse mythology as I'm sure it was to so many. Classical Mythology demonstrates unequivocally that the canon is a process of multiple artists riffing on the same characters and themes.
Archaic mythologies were almost certainly oral tradition before they find their way into text, and certainly remained in the oral tradition afterwards, as most people were illiterate until fairly recently.
I've seen questions on the Grimm's work, which drew heavily from folklore, but also probably contains much that they invented. Even if we don't consider Lovecraft to be proper mythology, it has certainly become legitimate folklore, and has had influence on later work consistent with traditional mythologies.
But aside from the philosophical point, I bring this up b/c if traffic is what we want, allowing popular, modern mythologies is one way to get it.
I recognize that this may be a Pandora's box, but a way to limit which modern mythologies are allowed is that they have had to significantly influence subsequent work that is generally known. (Thus, Spagetti Monster, Longcat/Tacgnol, and the mythologies of Essos and Westeros don't make the cut.)
Tolkien might be similar to Lovecraft in that his work significantly influenced an entire popular genre. I wouldn't necessarily allow question on subjects such as the plot or characters in "Lord of the Rings", but discussion of the underlying mythology, which is particularly well documented in the Silmarillion, would be consistent with the type of discussion and scholarship we're tying to encourage.
It's also worth noting that neither Tolkien or Lovecraft were merely pulling these ideas "out of their butts" but both were, in a very real sense, trying to reconstruct lost mythologies--that of the Elves for Tolkien and the elder, pre-human gods for Lovecraft (recall that redemption is a fairly recent concept in human religion.)