I've thought that it would be cool to see someone that he knows, like Jenny, as another full girl in the house. I think that would be quite interesting, how unlike the other girls who have to adjust to the trappings of our world, she would be approaching it from the opposite direction, she'd be intimately familiar with our world but would have to adjust to that dork she knew in high school turning out to be an extremely powerful wizard. I don't think that we are getting any more main girls though.
I can't really see how a girl from our world would be "competition" for Rae though, it's a harem game there is no competition. Even when the girls fight with each other, there isn't any hints of jealousy.
Rae is always playing the role of queen, or lady, or headwife. The others, so far, are quite far from suitable to this role. If someone else fitting the role of headwife, there could be competition.
Speaking of harem, I believe in history there are at least two kind of harem. The very word harem comes from Ottoman I believe, where the wives of the sultan is mostly some slaves from god knows where, with basically no rights. Most of the time there wasn't a queen or something, and the mothers of the sultans are often debated. Another kind of harem is in east asia, especially china. The wives are usually from powerful families, and there was almost always fierce, almost all out, competition for the role of empress, and the mother of future emperor.
My point is, it is probably quite reasonable to expect some competition in harem (unless the man has absolute power). It is just dev may or may not include that part in a game.
ZanithOne