OKAY SO
Human!Arceus comes in with Bby!Raihan (not actually baby Raihan, but Raihan who isn’t a gym leader yet for Galar) and sets him in the chair opposite of Lance.
“Who, what, where?” Lance ask’s I’m so much confusion, but Head Council Ace (Human!Arceus) ignores him.
“I need to train Raihan here.” He states, patting poor scared Raihans bony shoulders and fucking leaves.
Lance is immediately like “am I getting fired!?! Can they fire me!?!?!” But he’s calm when he faces Bby!Raihan. Who’s fidgeting up a storm because he’s in his idols presents!!!
“What’s your typing?”
“Dragon”
Lance: well S H I T
So Lance starts training Raihan, and if he drags the other Dragon type gym leaders/Elite Four into it than that’s his choice, and while he’s there Raihan meets Bby! Red and Bby!Gold who are now Champions of Kanto and Johto respectfully.
So that’s also a reason why Ace gave Raihan to Lance so the Elite Four member could train the kid. Lance doesn’t know what to do with his time now because he’s so used to 4x’s the paper work for being both an Elite and the Champions of the two neighboring Regions.
And where ever Red and Gold go, the gang follows.
Raihan ends up meeting Silver, Crystal, Green, Daisy, Yellow, and OFC actual Baby Ash and Gary. He also meets their parents and Delia was so close to adopting Raihan into the family because the boy didn’t have his on family since his dad was apparently to “busy” to come and raise his son and Raihans mother died during child birth.
So he comes back every summer to train before going back to Galar for the next five years, then the next five is to just hang out and rant about the Galar League and have fun.
Idk about age so far but here’s the final ages for a few of them.
Red - around late 29 to early 30
Green - same as Red
Gold - 26 to 27
Silver - 25 to 26
Crystal - 27 + 5 months on Gold
Ash - around 23 to 25
Gary - same as Ash + the 6 Months he has on him
Raihan: - around very early 25 to very late 27
Daisy - 35 to 40, she just needs to be a few years older than Green and Red
Lance - around 50 to 60, but he’s emotionally an 80 Y/O with his wisdom and what he had to put up with the Ketchum/Oak kids and the other National Poké Dexers
Delia - 50 to 57 she’s needs to be older than Daisy, but younger than Lance
Ace - he’s fucking Arceus, does it look like I know his fucking age!?
Lance soon finds out that, No, Raihan isn’t immediately replacing him and that he’s being trained in stead because the current Galar gym leader at that time was finally sub-coming to her Cancer. So Lance was the next option because he’s been the longest Dragon trainer in the league after her.
(Hint at that Immediately??)
Over time Raihan soon sees that the other Gym leaders are drawing away from him, Chairman Rose and Oleana are now piling more and more paper work onto him, and now Leon and Sonia are now avoiding him at all costs.
(I haven’t figured out a reason why just yet, but I’ll get there)
And now Raihan goes to the council, because he can’t go to Rose because he knows how absolutely shitty he is about complaints and other things, and begs them not to actively search Rose until the Council is done with their search of evidence.
Unbeknownst, Ace has been gathering shit on Rose for Years now. So he uses this as a chance to freak the head chairman out. 
So Ace scheduled for him, a few other Council Members, Lance, Gold, and Red to come to the next beginning League ceremony for the start of the Season. Now Chairman Rose is internally freaking the FUCK out because he has council members and the three most terrifying International league members coming to watch the Galar League opening ceremony.
( Ace also starts hounding Lance, who’s hair is now mostly grey now with red streaks, to start looking for people so he can retire. Because Alder’s position got taken by Black and Both Alder and Drayden are wait patiently for Lance.)
(( and if Ace also can get Raihan to take both Lance’s Elite position, while also Raihan being closer to Ash and the found Family that’s been yearning for the dragon tamer to join them permanently - via marrying Ash finally - then that’s Ace’s second main secret ))
Rose sticks them down with the other gym leaders because he doesn’t want to sit near the council members or other league members. But that also means he gives them a great chance at “talking” (more like interrogating) the Galar gym leaders
And they find out that they have been distancing Raihan for multiple reasons that are pretty fucking weak. Ace pulls Raihan aside and gets the full sob story and then gently asks if Raihan would like to move to the Kanto League, which has grown substantially stronger with Red, Gold, and Ash at the helm of those Regions.
And Raihan subconsciously melts at the thought of being closer to his found Family and his Ash. He answers hesitantly that if they have a position, than Raihan would sign all the paper work today and that he already has several people ready to defend the Vault in his stead. Raihan then gets pulled away by a very nervous assistant that says it’s time for the final battle with Leon.
Ace joins up with the others and starts to casually grill Lance that Raihan would be the best person to fill his two Elit spots.
“What!?” Lance looks shocked at that thought. And Ace tries not to laugh at his God-nephew face.
“You and the rest of the Dragon Tamers trained him. And I also noticed how you subconsciously trained him to take an elite four position instead of a gym leader one.”
“That - that doesn’t mean anything!”
“Everyone that he needs to know has already met him and like him. Unlike the others you’ve found where they don’t like several people in the line up.”
“S-so!?”
“Lance, look at him! His own Gym uniform is styled after your outfit! He fits well with everyone, and is just as strong-if not stronger than you. He’s completely perfect to take over your position.”
“...”
“Hell! Chairman Rose is already pushing most of Leon’s Champion paperwork onto him, creating just the same amount-if not, again, more paperwork than your two positions.”
“I... do see your point.”
“So?”
“...”
“Lance. I’m not talking to you as the Head Council, your Boss, or an equal League member at this point. I’m talking to you as a concerned family member an as your God Father. It’s time for you to rest, Your Husbands are waiting for you and you’ve denied yourself this for a longer time than absolute necessary.”
“But the kids-“
“-they can come to you! You are still there! Available when they need help or an answer only you know.”
“I don’t want to spring this on Raihan though.”
“And we don’t have to. You can still train him as we bring in another gym leader, I’ve been gunning to put a bug gym leader in Galar anyways.”
“But-“
“Lance, Watashi no musuko. Stop making excuses, the future generation of League members are here. Right in front of your face. Just like when you first came Champion of Kanto and the rest son followed after you. You’ve been Champion of Kanto for over 30 years, Johto’s Champion for over 25 years, and Elite four for both for over 28 years.”
“...”
“Watashi no musuko. It’s time to rest.”
“...”
“Okay?”
“Okay, I’ll train him.”
“Good. Thank you.”
“Thanks for telling me to cut my shit.”
“I’m not the only one, Delia will hound you now more than ever to come to the family dinners.”
“Ugh! Don’t remind me!!!”
It all boils down to Raihan getting moved over to the Kanto Elite four under Red and finally start dating Ash. Chairman Rose gets taken down and Raihan is now allowed to heal.
————————
Idk this thought has been plaguing my thoughts for the last couple of days and I just had to wright this all out.
I think I’m calling this the Replacement!AU