Here's what I think:
1st Gym: Homika. Venipede Lv. 13, Koffing Lv.14. Her Gym will be in the second town. Gives Venom Badge and TM09 Venoshock.
2nd Gym: New Leader. Hopefully a Dark type. His Gym is in the new, big waterside city. Pawniard Lv. 17, Vullaby Lv. 19, Mightyena Lv.20. Sorrow Badge and TM95 Snarl.
3rd Gym: New Leader. A teacher at the trainer school in Hiougi City, so you backtrack. (The teacher originally says you're not experienced enough.) The school is the Gym. Leader is interested in fossils. Rock type. Archen Lv. 22, Tirtouga Lv.22, Larvitar Lv. 24. Rugged Badge and TM23 Smack Down.
4th Gym: Clay. Ground type. Clay returns as Leader of Driftveil. Sandile Lv.26, Palpitoad Lv. 28, Drilbur Lv.29. Not sure how to explain what happened to his old Pokemon. Still gives the Quake Badge and TM78 Bulldoze.
5th Gym: Skyla. Flying type. Skyla returns, too. Gym is still in Mistralton. Swoobat Lv. 30, Tranquill Lv.31, Ducklet Lv.32. She's matured, and has a less skimpy outfit. She's wearing a thick leather jacket, a hat like
this, and boots. She gives the player the Jet Badge and TM62 Acrobatics.
6th Gym: New leader. Opelucid City. Dragon type. Drayden has died and Iris has returned to her home village. The new leader is Drayden's niece, or some relation to Drayden. Gabite Lv.35, Altaria Lv.36, Fraxure Lv.38. Gives Legend Badge and TM02 Dragon Claw.
7th Gym: Shizui. Water type, of course. Basculin Lv. 39, Gyarados Lv.42, Ludiculo Lv. 44, Sharpedo Lv.45. Fluid Badge and TM55 Scald.
8th Gym: Elesa. Electric type. Nimbasa City. Emolga Lv.48, Zebstrika Lv.49, Ampharos Lv. 51, Galvantula Lv.52. Bolt Badge and TM24 Thunderbolt.
Anyway, just thought this would be cool.