I'm kind of positive, Toad. Kanto and Johto are both pretty small on their own, and building a game on either alone wouldn't cut it anymore. Probably why the Sevii Islands got added to FireRed and LeafGreen. I mean, going to Kanto was never about catching old Pokémon. It was about... going to Kanto. Besides, there is no Pokémon League HQ in Johto. It's still Indigo Plateau.
No, on the contrary, I bet Kanto is bigger. It was cut a lot in G/S/C to make the whole game fit in the cartridge... but that's not a problem anymore. There are several locations in Kanto that can be used for Battle Frontier landmarks, like that building in Vermilion that was never actually built, or Mt. Moon Square. The Safari Zone will probably be open. I wish Cinnabar Island would get rebuilt after you first defeat RED, but who knows. Rebattling gym leaders is a must--BLUE was my favorite and he promised GOLD he'd beat him someday...
On this topic, I remember walking into Ruby thinking "I wonder how much later than the other games this takes place. I wonder what Kanto and Johto are like now!" The first indication that this wasn't exactly going to be an eternal pattern was the absence of Rattata and Pidgey... The original trips through Kanto and Hoenn are supposed to occur simultaneously, and the trip through Johto aligns with the one through Sinnoh. That being said, I'd have paid for and played Platinum if D/P/Pt sent you though an aged Hoenn.