If we're talking looks wise, then Kanto is already up-to-date(before we go full 3D with X & Y) with the region itself being included in HGSS. Hoenn (itself) has yet to be caught up in this sense.
Only real Kanto is from the Gen 1 Games. Gen 2's Kanto is a shadow of the former form. As for Gen 2 Pokemon. Only Ho-oh and Lugia were unavailable in Gen 3 main games. The two box legends of Gold/Silver. Not a real enough reason to make two whole games, but it was done and they were added into distribution through main games.
Its game was removed from the circle, that alone is enough for them to add it back in since their whole shtick is connecting games with others.