Soooo would Amity Park basically become like a city-state? They isolate themselves away and work to protect their own from those that might do harm.
They work to make additional portals as alternative escape routes and have bunkers and an underground network for getting to various evacuation points (both through the GZ and otherwise).
Tucker and Technus make the city network invisible to anyone outside of it and impossible to hack or otherwise invite yourself into it.
With time the Fentons only hunt the ghosts that want to cause harm and work on creating non weaponized ecto-tech. Scientific advances (Healthcare, transportation, technology, etc.) leap forward by several decades for both ghosts and humans and Amity Park becomes completely self sufficient.
The citizens of Amity Park that keep an eye on the news realize that the "Justin-Leakers" (who they know are the Justice League but they enjoyed seeing their faces so much the name isn't changing within the city limits) have their own rogue gallery they go up against regularly- some of which seem to just need certain accommodations to live relatively peacefully. After all, most of their GZ residents don't visit to destroy but to fulfill a need -whrther that be an adventure, a concert, a game of chase or hide and seek, or to add to their collection. So they understand a bit more than most people the consequences of keeping someone from being able to fulfill that extra need.
This can then mix with that one post I saw somewhere (can't remember where) where the JL find their own enemies disappearing only to find they've moved to Amity Park.
Poison Ivy and Overgrowth join Sam in charge of the city's green spaces and assisting with the greenhouses. They also have a selection of seeds around the city boundaries just in case. They also help to produce various medicinal herbs and spend their free time testing out various hybrids. There's a poison garden they can play with that also allows them to develop anti-toxins and antidotes.
Jazz works out deals and makes arrangements for Harley Quinn and Spectra to allow them to heal and help others as well. Spectra won't work with teenagers but is fine with kids (there was a fic where she's fine with children and I loved it! I read it a while ago so I can't remember the name. I'll link it if I find it again), and Harley's not a fan of adult men. Jazz works with everyone and is thrilled she has people to talk about psychology and behavior with.
They don't take all the rogues - just the ones that have been wronged, or didn't have an outlet and were pushed to far. The ones that were made by circumstance and misguided aspirations rather than by choice and a lack of care.
Amity Park becomes "the myth of the mid-west." "A mini Wakanda"- invisible and hidden to the outside world, protected by their own. A "chaotic paradise" - a "modern pirate isle" that seems to abide by its own rules. A city inhabited and run by the fae.
Also- i kinda like the idea that there would be some JL/YJL members they accept/adopt...potentially forcefully. It seems random until you realize its those that the citizens feel are being used for their power or only allowed to be a member so there's a way to keep an eye on them.
When Dani learns about Roy and Connor she snaps them up so fast. She claims them as hers and brings them back so they don't feel like their lives are predetermined or dictated by who "the original" is. Bc she knows the people of Amity Park won't care (especially Danny). And they can learn to be comfortable being their own person.
Jason is another forcefully adopted member. The fentons work with frostbite to figure out the level of ecto-contamination and whether he's been getting enough ectoplasm to stay healthy (ecto-hangry Jason AU). The other ghosts help him figure out what his obsession is. If it's an unhealthy one they work out how to tweak it so it can be fulfilled/acted on in a healthier way. He joins the spars to blow off steam and for the first time in a while he doesn't have to hold back. He joins the action squad for whenever the GIW tries to come into town. But otherwise he enjoys working with Ghost Writer and the town librarian - making reading programs for the kids and running the various book clubs.
Billy (Shazam) is adopted rather quickly bc the ghosts would know who he is and see him as another baby hero they have to protect regardless of the age his change makes him look. I feel like he and Danny are the semi-oblivious babies of the group. They're encouraged to work as heroes (they're not going to stop them) but also they take care of a lot of things from the sidelines and in the background to make sure they're not in over their heads.
Arthur (Aquaman) and J'onn (Martian Manhunter) are allowed to visit and act as emissaries when the ghosts find out they're ready to throw hands with the govt and GIW to protect the ghosts. They're the only league members they semi-like but the agreement is no league talk/business while in city limits, and if they show up in costume they get the same treatment as the rest of the "Leakers".
Aquaman to Martian: So Jazz and Phantom were complaining this weekend about how-
Rest of the inner circle for the league: wait the people of Amity Park talked to you?!
Aquaman: no you idiots. Remember? No JL members allowed? They yelled at us pretty loud to go away. Thought it'd be easy to remember something like that. [Back to Martian] anyways, so she's complaining about how the combo of Pamela and Overgrowth have made this pollen season really hard and-
Rest of JL (especially batman who's been worried about the lack of active from Poison Ivy bc it means she's planning something) :?!?!?!?!?
Various members of the JL coming by the city and seeing one of their rogues in the background: There you Are!
JL: [insert villain name]! Why are they here? They're a villain!
AP: [dramatic looking confused] sorry we don't have anyone here by that name.
JL: they're right there! [Gestures]
AP: oh! You mean [civilian name]? They've been here a long time. Family's been here for generations [shows fake papers to prove it, thx Tucker] so there's no way they're your [villain name but wrong on purpose] or whatever. Just not possible.
AP, [pushing them back out the figurative door]: so thx for stopping by but you're not wanted nor welcome. So you can go now. Bye!
[Slams figurative door in their face]
I'll stop now bc this is already really long