The thing is, Gary (at the front) played a semi-important role, as he provided Ash with the Razor Fang, for evolving Gligar. May (in the middle) played a smaller part, as she was Dawn's opponent but didn't really contribute hugely to the ongoing plot. So how small will Misty's role be if she's at the back?
Well, technically, it did contribute immensely, since their battle was what made Dawn get back to a winning streak, and boost her confidence to high levels. (Of course, I'll still hate May, and never will respect her as she is of that... f... a... s..., but still.).
Besides, one can argue that Gary played a smaller role as well, as literally anyone could have gotten Gligar to evolve by supplying Ash with the Razor Fang.
As for the smaller role of Misty, we won't know, yet anyways. For all we know, she might in fact return to the main cast (I mean, it's not just her position in the opening, The mere fact that Mayumi Iizuka would actually hint in her blog at an appearance that she didn't do in any of the previous eps that Misty appeared in (including Mastermind), AND confirming the narrations of the quiz, for that matter, implies that Misty's in for something big in Sinnoh. I mean,
I hope that Misty will return for a significant amount of time, at least fo a mini-arc. I hope that her time on the show will be significant. But knowing the writers of the show, she'll probably get one short, meaningless episode before being forgotten until we reach next generation. And that's if we're lucky.
True... but, then again, Misty's Seiyuu, Mayumi Iizuka, did say this in regards to Misty appearing in DP:
According to ☆飯塚雅弓のまーちゃん日和☆, the blog of voice actress Mayumi Iizuka, Kasumi will be "navigating," that is to say, hosting, parts of the first Pokémon anime 10th anniversary special, which airs on TV Tokyo tonight at 7 p.m. JST. She hints at a further appearance sometime in the future.
Now granted, this announcement was made at least two years prior. However, she only hinted that Misty might appear sometime in the future, she didn't necessarily say it was "soon".
Besides, during Twilight Princess' development, something similar happened. Nomura had stated in an interview that Ganon is going to be appearing in Twilight Princess, and he will be more than anyone could possibly ever expect. Despite it taking three or so years between that announcement and the release of the game, not mentioning Ganon again after it, and people being split up on the issue, Ganon, did appear in that game.
@KnilShadowLink: Well, I never personally believed in Pokeshipping (mainly because of how I've been influenced by certain media into thinking that the only way to fall in love is through Love at First Sight.)
@SirBoulevard: The main reason we couldn't identify him was because of that April fools day prank that Pokecommunity placed onto the site.