There has been an ongoing thread on 4chan that is focused on rescrambling species to create regional dexes, so here's a grouping list for existing starters:
Bird-based: Decidueye, Blaziken, Empoleon
Draconic: Sceptile, Charizard, Feraligatr
Field(Feminine): Meganium, Delphox, Primarina
Field(Physical): Chesnaught, Incineroar, Samurott
Defensive: Torterra, Emboar, Blastoise
Speedy: Serperior, Infernape, Greninja
Bulky: Venusaur, Typhlosion, Swampert
This seems a decent enough selection. I'll probably rescramble it with the next generation.
Edit(22.10.2017):
Some possible additions would include:
Egg Group Based:
- Bird-based Grass-type starter for Field.
- Bird-based Fire and Water-type starters for Flying.
- Draconic Water-type starter for Dragon
- Draconic Grass-type starter for Monster
- Feminine Grass-type for Field.
- Feminine Fire and Water-type starters for Monster.
- Fire-type turtle.
- Field-based Grass and Water starters for Emboar.
- Field-based Speedy Water-type starter
- Bulky Monster-based Fire-type starter.
Pseudo-legendaries, potential stats:
- 105, 85: HP
- 125, 120, 115: Attack
- 120, 115, 105, 100, 85: Defense
- 120, 115, 105, 90, 85: Special Attack
- 145, 140, 130, 125, 120, 115, 110, 95: Special Defense
- 95, 90, 75, 65: Speed
Edit (26.10.2017):
-Early Electrics have been so far:
- Field, Fairy: Pikachu
- Field, Monster: Mareep
- Field: Electrike
- Field: Shinx
- Field: Blitzle
- Monster, Dragon: Helioptile
- Field, Fairy: Pikachu, again.
Honorary mentions: Mineral, Alolan Geodude, Bug: Grubbin
Monster/Fairy and Fairy/Dragon are unused Egg Group combinations. Field/Dragon is used for snakes and Scraggy. An electric snake, maybe?
-Water/Ground species:
- Water 1/Field: Wooper
- Water 1/Monster: Mudkip
- Water 2: Whiscash
- Water 1/Amorphous: Shellos
- Water 1: Tympole
The sole Water2/Field member is Wailmer, so a Water/Ground whale maybe? Monster/Field is another large group. Water2/Water 1 can occur, maybe a cuttlefish?
In terms of Pseudos, there are no Water 1/Monster pseudo. Possibly a Rock/Water, Water/Flying or Ground/Water.
Edit(27.10.2017):
Dragon/Monster exists for Fire and Electric. Grass, Water and Ice can get it.
Edit(28.10.2017):
There are no Ice/Ghost types in Amorphous Egg Group, yet.
Edit (29.10.2017):
Going on some potential counterparts to Ultra Beasts, here are some options I have:
- Cyberspace species: Similar to current Yu-Gi-Oh! series' Cyberse monsters, I thought of a sub-group of species that will be encountered in the cyberspace. These species would be two staged species with stats based off composite numbers, as opposed to prime numbers. Similar to Nebula trio, the respective main legendaries would be based off these cyberspace species, with third legendary serving as a Minicon style upgrade.
- Time-displaced species: Similar to UBs, these would be species that are displaced in spacetime. Future variants would be Steel and Electric focused, Past variants would be Rock and Psychic focused. Some would have version dependent side evolutions. The third legendary would "age" the main legendaries.
- Rare migrating species based off different seasons can work. Third legendary would force the given season upon fusion.