The writers had two opportunities to show Janine, the Kanto Gym leader who takes over after Koga gets promoted to the Elite 4, in the anime in the past.
The first time was of course in Johto. Instead of bringing Aya back, they should have had Janine in that episode. It would have been nice to see her training to take over Koga's Gym. But she was ignored and didn't appear.
The second time could have been the Battle Frontier saga. It might have been interesting to see Ash stop by Koga's Gym, only to see Janine was the Gym leader. But it didn't happen there either.
Now with HGSS coming out, it would be nice to randomly see Janine in an episode even in Sinnoh. We saw the writers suddenly introduce Wallace when most thought we'd never see him, so it would be nice to see Janine. Janine is the only older Gym leader that has never appeared in the show.
The first time was of course in Johto. Instead of bringing Aya back, they should have had Janine in that episode. It would have been nice to see her training to take over Koga's Gym. But she was ignored and didn't appear.
The second time could have been the Battle Frontier saga. It might have been interesting to see Ash stop by Koga's Gym, only to see Janine was the Gym leader. But it didn't happen there either.
Now with HGSS coming out, it would be nice to randomly see Janine in an episode even in Sinnoh. We saw the writers suddenly introduce Wallace when most thought we'd never see him, so it would be nice to see Janine. Janine is the only older Gym leader that has never appeared in the show.