→Generation IV with Generation II?
:Hoenn and Kanto take place at the same time is a weak bet, but it's mostly because you don't need a time machine to trade between the games. Therefore, they must trade at the same time. Gen 2 and 4 are contemporary because of: A. the Red Gyarados report at the start of DP (which must happen before Gold/Crystal captures/defeats the Gyarados), B. References to Elm's RECENT report on eggs, and there are more, but those are two of the big ones. Persumably, it takes 3 years or so for Pokémon to reach Sinnoh (Assuming by land, on foot) from Hoenn or Kanto.
At the beginning of RBY, you have a Nes/Snes. At the beginning of RSE, you have a Gamecube. That's pretty odd, because the NES and Gamecube were released more than 10 years apart, how could they take place at the same time? The time machine doesn't necessarily indicate that RSE takes place at the same time as RBY, because since the data for pokemon was available in both FRLG and RSE, then you could freely exchange pokemon and wouldn't need an excuse to only transfer certain pokemon
== Gym Leaders ==