[Locksmith]

Sorry, being a bit thick - what do you mean by 'mainstream', given that the most popular books of recent years have been fantasy (even if not acknowledged as such by their author)? If you mean not in a genre, haven't you just defined away your question - i.e. there can be no crossover, it's either genre or not?

Anyway, pedantry aside, I'm not sure this is a terribly novel question, although it is interesting. Starting from the premise that all fiction is, by definition, make believe (else it's non-fiction), the author decides whether to keep all of the elements of the real world, or to tweak bits here and there. Depending on how much the author decides to tweak reality, you may get deeply into a science fiction or fantasy genre. If they don't change things much, it may be what you are calling 'crossover'.

It's perhaps easier to see the relation between the more marginal science fiction and the real world, because some science fiction says "the world could be like this in x years time" and we can identify with that. With fantasy, a lot of the genre is based in different worlds and is therefore seen as being totally different. Sword and sorcery is what many people call fantasy. But the more subtle fantasy is often seen as part of mainstream fiction or historical fiction or horror.