I assume Misty is still running the Cerulean Gym.
Brock might still be studying to be a doctor by travelling through different regions.
I'm guessing May had finished her contest quest in Johto by the time Ash's journey through Sinnoh ended. Unless May ended up winning the Johto grand festival and became top coordinator, I assume she was journeying through an unknown region while Ash was in Unova and is now in another unknown region during Ash's Kalos journey.
Max may be training to become the next Petalburg gym leader or has gone to the trainer school in Rustboro.
I'm guessing after Dawn finished her Hoenn journey during BW, she moved on to Johto (she did say that she was heading there for a Wallace cup during her departure in BW2).
Iris and Cilan are probably still in Johto throughout Ash's Kalos journey or Iris might've headed to Hoenn after her special episode since she wanted to follow Rayquaza and there are a lot of dragon types in that region. She probably also went there to challenge Drake.