I'm going to post my planned teams for each Gym Leader and I'd like you to give feedback. I'm trying to put myself in the eyes of 1999 GF and think what they would've done if they had implemented Gym rematches at the time. I want this hack to feel as vanilla as possible. I'm also trying to keep each team as close to the user's theme as possible, and avoid power creep (no Legendaries or anything like that).

All 8 Johto Gym Leaders and all 8 Kanto Gym Leaders will be rematchable via registering their numbers in the Pokégear after beating them for the first time, and it will work similarly to regular Trainer rematches; you can call them on specific days/times to offer them a battle, and they will also call you randomly to offer you a battle. I'm also going to add Elite Four + Champion rematches, but that won't be via Pokégear, they will just be higher levelled and use full teams after you beat them for the first time.

Let's start with Falkner's rematch teams.

Falkner - Initial battle
Falkner 1 - After reaching Ecruteak City and defeating the previous team
Falkner 2 - After the Goldenrod Radio Tower incident and defeating the previous team
Falkner 3 - After becoming Champion and defeating the previous team
Falkner 4 - After returning the Machine Part and defeating the previous team