Ooh, this is a difficult one!
Put it this way - which region has the most to do? Kanto and Johto were simple and you could get around to where you wanted easily, but there were side-quests as well. With the Sevii Islands for LG/FR, you had a lot more to roam about in. Personally, I think that they qualify as in the Kanto region, since they're obviously alot more close. However, in Gold, Silver and Crystal, as well as Johto, there is Kanto. This is two regions, meaning that the game has more. Move up a generation, and we have Hoenn. A fairly big region, however, no other regions to go to. Apart from Faraway Island, Navel Rock and Birth Island, there's nowhere else. Then Emerald came out with the Battle Frontier, and this was a very nice thing. Move up another region, and it's Sinnoh. Again, no other regions to go to, it was slightly more boring than Hoenn. However, with the Underground and the Wi-Fi connection, there's alot more possibilities!
A possible speculation here, in HeartGold and SoulSilver, with the ability to go to Kanto, who knows, the Sevii Islands will be back! Only time can tell.
So, to answer the question, Johto. Ability to transverse to another region, and possibly, the ability to go to the Sevii Islands!