If someone asked me to make a Pokémon fangame (or maybe even an official spinoff) and I had the time and resources, I'd love to make one based on Team Magma or Aqua succeeding. One game would have a lower sea level, traversable sea routes by foot and an industrial theme. The other having a higher sea level, expanded dive routes and have certain areas more densely populated like Pacifidlog Town (City?). Just want to put that out there.
The same idea as mine, but what I find most interesting about this concept isn't the pokémon gameplay per se but the socio-enviromental part of the hoenn accident.
In my idea it would be a sequel of 8 years after the events (you as ruby/sapphire returns to hoenn after failing to stop the aqua team, you never completed the 8 badge challenge, wo you returned to johto with your mother as the gym leader father stayed on the flooded hoenn)
The lower levels and coastal cities would be flooded and this caused a big societal rift into people hating pokémon and other problems like xenophobia (a person with hoenn accent would be shit talked and treated badly in other regions since the rise of sea level affected them too, this was inspired by the Hibakusha in actual history from the atomic bombs in WWII.
Other concepts would be safe routes for surfing maintained by coastal guard police, groups of pirates trying to steal people on the seas, pacifdloge expansion, the exodus from hoenn to other regions and people wondering if they should raise their children in this waterfucked hoenn at all. Sootopolis city would become something like the 70's-90's city of kowloon in China by building around the mountains and going up and up covering the overall view from the sun in the water. Having some jobs to dive and recuperate itens in flooded cities like slateport, a city that is on a gigantic boat, magikarp farma intended to eating, the fallabor town would become a great agrarian city by producing vegetables. Fight for resources like drinkable water and river water in case of farms.
As for pokemon, new regional variants and evolution would surge, people would sometimes hate them or not, some would attack humans since they are also competing with resources now, making some pokemon hunters straight up killing pokemon in case of dunno, donphans raiding the farms. Also group of pokemon activists like team plasma acting retarded as they come from different regions and have no idea of how things changed in hoenn would be nice too.
The gym and league would of course cease to be in these 8 years but would resurge in minimal ways, instead of just fighting, the Gym challenges would be some minigame or puzzle to help a place in the area then you get to fight the gym trainer (some would be the same but older and some would be new) and maybe in lesser numbers like 4 or 6.
As for team magma, they would be considered heroes and help people.
If this game truly existed, the ruby/sapphire would have a totally new map and dex and would make sense to have two versions.
Maybe the big plot would use time travel/celebi to turns things back to normal or just improve their lives and accept the new reality.
It is a cool concept for me, and I would love to write these tidbits of npcs dialogue talking about these kind of stuff.