Full Power swap for Gil. Gil has a handful of powers, but nothing irreplaceable. Azula is capable of standing leaps of about ten feet, has incredible balance, is ridiculously strong given her build, has martial arts skills that would make anyone outside of a Wuxia setting blush, and has flamethrower/lightning bolt hands. An excellent baseline to build upon.
Full Power Swap for Charlotte. Send her to join either the wards or the local protectorate as a new pyrokinetic hero. It’s not like she has insufficient levels of trauma to justify a trigger event.
Full Power Swap for Mei. Turn little miss millstone into a powerhouse; she and Vahn can then proceed to handle that whole plotline in the lands shrouded in mist while Gil pops in occasionally to grab some magic items and Zena power levels by grinding in a JRPG.
Full Swap for Boulder, including appearance change. A keenly intelligent, manipulative, and powerful woman with a deep wellspring of self loathing due to her lack of ability to understand why those around her hate and fear her. Are we talking about the original Azula or the version that is a transformed stray dog)
Martial arts for Gil: Firebending is cool but not really irreplaceable in a world with fire materia. This will make Gil a master of kung fu with superhuman agility and exceptional strength
Martial Arts for Candress, eliminating her biggest weakness: melee conflict without magic to lean on.
Martial Arts for Rose. She can already fold most normal people like a lawn chair, but this would push her firmly into superhuman territory.
Martial arts for Snow. More for the massive spike in mobility than anything else, but even Snow is likely to employ mastery of kung fu if assaulted, even if only to escape.
Firebending for Gil. Flamethrower hands? Flamethrower hands.
Firebending for Candress. This will give her a form of magic she can use even when in dead magic (card abilities are automatically integrated) and a general boost to her physical fitness since Firebending is a very kinetic discipline
Firebending for Rose. Why wouldn’t we slap fire magic onto our shape shifting demoness waifu?
Save it for Flashbang