I could see Ash running a pretty epic Fire or Grass gym. Maybe even a flying gym.
Fire Choices: Cyndaquil, Torkoal, Monferno, Charizard
Grass Choices: Bulbasaur, Bayleef, Grotle, Sceptile
Flying Choices: Noctowl, Swellow, Staraptor, Gliscor, Charizard