Considering it's unlikely that Ash will be going any time soon, I'd say that weighs heavily on the possibilty of May going.
I'm already seeing a LOT of direct paralells from D/P to R/S, almost following the template in R/S to the letter. I have a nasty feeling we're simply going to get a replay of the start of Hoenn only with this chick being in place of May. :/
After the good run in Battle Frontier, i'll be disappointed if the anime writers drop all the way down to doing something as poor as that.
Gyms are obviously going to stay. They're the main staple of the games and if the current knowledge of D/P is anything to go by, they certainly aren't planning on innovating the template they made in R/S by replacing them with something else.
Being the hero and having the star of the entire Pokemon franchise living on his shoulder, Ash will not be leaving. It would be a marketing disaster for a show that exists to sell the Pokemon franchise - not to be all smart, arty or full of insightful character depth.
I'd consider it to be awfully crowded if she were to have her own side-quest alongside Ash and May, and it'd just get downright silly when it came to the end of the region - we'd spend half the year just getting through the various quest conclusions!
Personally though, I'd have rather they'd simply kept the current cast (though dropping Max, of course) and headed out to Shino. Though I can't disagree that a shakeup to make it slightly more fresh would be nice.
It doesn't look like we're likely to get any actual information on the new series until next month though, so I can pretty much imagine the next month is going to be nothing but arguments and flames from certain people getting all fanboy/girl about the prospect of their favourite character going.
I'll be really surprised if May gets replaced. I always thought her getting Munchlax guaranteed she'd be around for Shinou.
Then again, Munchlax got a ton of screentime in Kanto, so maybe it was supposed to be more of a pre-D/P taster. And May did just beat Drew, so arguably they could be trying to resolve her arc...
But no, I still think she's likely to stick around. Too popular. Perhaps they're going to shake things up and have two girls in the group. Or maybe D/P girl will be a recurring rival for Ash - that'd be cool.
I'd be awfully surprised if they really did wipe the slate clean for Shinou. The Battle Frontier was a great season, but there are still so many loose ends that haven't been touched upon with the current cast.
Of course, the lack of Ash, (or Brock or May or Max) in that poster may suggest we are getting something brand new. At least the first AG poster revealed showed us the entire group.
That poster, however, was from a massive cockup that revealed everything long before it was intended.
Everything in the run up to D/P has been a LOT more closely guarded than it ever was before R/S. It's really rather likely that this image was created with the sole purpose of saying "Hey, look, the D/P chick's gonna be in the anime! But be damned if we're telling you any more than that YET! ". I mean, there's really nothing in that image. Diaruga and Parukia show that it's for Diamond and Pearl, the girl shows that she's going to be in it. But that's it.
They aren't even anime images, just sugimori art plastered together.
Yeah, good point. Though in some ways the current cast getting their last hurrah in Kanto, the region it all started in, would have been quite fitting. Eh, I'd rather see Ash and co. continue on, though.
I don't think AG will be wrapping up intime for September 28th. We already know all the episodes up to AG186 (August 20th). That would leave about 5 episodes till this new series airs. It is most likely that AG will be postponed on the 28th for the new series, but I think it will air along with Pocket Monsters - Pearl and Diamond.
Satoshi-tachi journey, in my opinion, is far from over. I also could not see Satoshi be replaced as he is the anime series. It will most likely be a spinoff series.
There's plenty of time to wrap up AG before the 28th September. Assuming that there is to be a third and final installment of the Battle Pyramid, it could be directly after the Rivals Forever episode or after a filler between the two. Which at it's latest, takes us up to the first week in September and leaves us with 1/2 fillers on the way home to Pallet (Which shouldn't take too long being around that area and all) and then at least one episode there for Ash to decide to go off to Shino.
That brings us right up to the week before the 28th, meaning that the new series could easily start the week after that, on the 28th, just like AG did.