Honestly I wouldn't mind having such a region if they were to have you revisit the older region if it was the second game in a generation.
Electric/Fighting Sabeleye – Similar to Chargestone Cave, there's a cavern full of electrified crystals. The Sabeleye that inhabit these caverns have fed on the electrified crystals and have become hyperactive fighters. They have yellow skin with a pair of electric blue eyes. These Sabeleye can't Mega-Evolve but can evolve normally into a new pokemon.
Goblightning – Electric/Fighting – The evolution of this type of Sabeleye. Most if not all are hyperactive and are usually seen fighting each other. It's common for trainers to be approached by Goblightning who want to fight these. It's name is a play on Goblin and lightning as Sabeleye are based on the Hopkinsville Goblin.
Water/Electric Piplup line – The Empoleon have adapted to the warmer climate by preferably swimming around rocky areas to avoid predators. Due to either hunting or interacting with Chinchou and Lanturn these pokemon have gained the electric type.
Fire/Ice Tepig line – These Tepig are similar to Swinub are they are ice based pigs that's where the similarities end. Similar to Crabominable. They are based on Yeti's. These variants of the Tepig line are usually seen in mountains.
Grass/Fairy Chikorita line – Due to the lack of poison types. The pokemon in the Chikorita line have gained the fairy type to fend off from predators like bugs.
What ideas do you have?