Well first i'd focus that gen on elaborating on the pokes we already have, making very few new pokemon, maybe about 50 or so, most of which would somehow develop those we have, such as a Lati-baby that can only be made if you have both Lati and breed them.
For example i'd give every single one of them a new dex entry, and have the people of this new region live more together with their pokemon, like some sort of hippy culture, or Pacifidlog. Some places you'll also stumble upon packs of certain pokemon in the overworld and see them in their non-hostile behaviour.
Every pokemon would now have two abilities, one for those that is only logical, such as Levitate for Magnezone, while it would still have either Magnet Pull or Sturdy.
Also give some more variations such as implementing all Arbok's different patterns, and then some, giving Shellos another two variations.
Also some pokemon would have variations that somehow work differently that you can only catch one of, for example an overgrown Pumpkaboo, has the same ammount of bst, but taken from SpA to add to HP.
The Bad guys wouldn't be trying to capture any legendary, instead, they'd steal powerful gems or something from around the region to power these destructoid machines they are making. When they are finally done, you have to fight some of them in a battle similar to Pokestudios, Last battle is against their huge ship that has more than 4 moves, some of which can not be learned by pokes.
also this and similar problems
>>22876101 
