Hmm, interesting theory. I would agree on most of that, however... I don't think it will end after that season. There is confirmed to be another Generation of even more Pokemon, and they won't quit till it's over.
But it's all a matter of time. Johto started in Japan the day GSC was released there, but Kanto finished a bit earlier. Therefore to take up the time, they had the Orange Islands season, which was basically a random unpredictable promotion of all the RBY Pokemon, with some more GSC sneak peaks like Marill. They timed it to end the day GSC was released.
R/S was released in Japan the day Hoenn started there. Johto, however, was an ENORMOUSLY long league, that they had enough time to span it across and time the release of R/S all the way to the end of Johto. I believe this is the reason they did this, cause they couldn't come up with an Orange Islands type season to randomly promote all the RBYGSC Pokemon while we were awaiting R/S. Johto was long enough to cover all that.
That would mean that the next season based on the next generation Pokemon game would start the day it's released. As some of you may have seen, they're advancing through the gyms pretty fast, and most of the episodes in between have a point. The few fillers are mostly really good fillers. So, at this rate (kinda a spoiler but not really, since it's obvious this will happen) in Japan they're near the 5th Gym already, and we haven't heard ANYTHING about a single one of the new Pokemon. Therefore, I agree there will be another OI type season, with random unpredictable features of Pokemon of all three generations, for the meantime we await the release. A return to Kanto would be nice, but it would be kinda tough to smuggle some non-RBY Pokemon in there without making it seem enormously bad... but given the Hoso's, it's possible. However, most of the Hoenn Pokemon featured there seem to belong to some trainer. Then again, Sakura gets a Beautifly and she's from Johto. Then there's that Delcatty/Pelipper trainer, that's near Cerulean City... he may be traveling though *shrugs*. Oh, and then there's the guy who makes Cascade badges in Rifure Town (an Anime-exclusive city in Kanto featured in one of the Hoso's, it's close to Cerulean), and he has a Feraligatr.
So... yeah, I say there's about a 30% chance of a return to Kanto, but definately not the end. I'm more for a filler unpredictable season between Johto and Generation 4.
Also, I'm gonna move this to Spoilers. I know this topic is based on speculation rather than spoilers, however, as I was posting this and making speculations based on Spoilers, I'm pretty sure there's a big chance that a lot of the replies may be based on spoilers. Hope you don't mind Ozzy
~Zak