I don't wanna sound offensive, but it looks like many people here simply are ashamed of getting caught playing a "kiddy game" and wish for a higher rating so that they can get out of the closet or play without being bullied. I mean, why is this thread titled "rated higher than E" instead of, say, "feature deeper/better storylines"? Ironically enough, giving so much importance to the rating is slightly immature to me when all that matters is the gaming experience.
I only care about contents, I don't give a shit about ratings, they say nothing at all, take for example, Majora's Mask, it was rated E yet it contains such emotional, deep and complex stories and is as dark as any other Teen rated game (actually, now that I'm on Zelda, it's way darker than Twilight Princess which did receive a T), which proves that we can have mature and deep plots without restorting to violence and swearing. Do I want a mature Pokémon game (something we already had with BW, although I think it can still deepen a bit more )? Yes, of course. Do I want a higher rating? I couldn't care less. For all I care, it could be rated EC (Early Childhood, does that one still exists?) and I wouldn't mind as long as the game is awesome.
To be honest, I think the ESRB (don't know about other rating associations in Europe and Asia though) is useless nowadays: its ratings don't actually measure maturity in games' contents, just explicit violence, language and sex: if a game has none of these, it's automatically E, no matter how dark or sad it is (take the Kirby games, usually they have very disturbing and scary final bosses, but since they star a pink puffy ball, it's automatically for kids, thus, E). If it features them to a mid level, it's T, and if it's abundant, it's M or AO. That's all. That's why I say it's immature to care about them: the only people who actually want or are happy that their games are higher rated, are kids who think they are hip because they play "mature" games aimed towards teens or adults. But come on people, we are above that, aren't we?
So no, the only way Pokémon can get a higher rating is if the series starts displaying gratitous violence and since that won't happen (and, if it happens, it would be for the worse), we can rest assured that it will never get a higher rating, and in the end it doesn't even matter.