I've had this idea on my mind about conceptualizing a tiro of starter, so i'll give it a shot, feel free to join me. These ain't no leaks, rather speculation/theorizing.

So to give some kind of guideline:
>The trio of starters should have at least 1 theme between each other. For example, in gen 6 the main theme is Warrior/Rouge(ninja)/Mage.
Any other "pattern" or "theme" you see fit is up to yourself.

Main themes Armor/Shield/Sword and mythical creatures
>Fire starter (Calf-->Bull-->Minotaur)
Armor (Fire-->Fire-->Fire/Steel)
A combination of the Sicilian bull, armor and a minotaur.
>First stage: basically beef that cooks itself warm to touch, doesn't release flames
>Second stage: a pre-adult bull with small horns and some armored bits, these patches get very hot and can be dangerous to touch
>Last stage: a minotaur with heavily armored patches and orifices in said patches that release flames when angered

>Water starter (Uca crab-->Xantho Poressa crab-->Carcinus)
Shield (Water-->Water-->Water/Fighting)
Crabs that carry a shield.
>First stage: a small crab that drags along a shell too big for itself, this is a Calico Scallop shell
>Second stage: this one carries a shell easily, but can't carry anything else besides it, it is a Sunrae Venus Clam shell
>Last stage: this is a a giant crab, easily 5'11', not an spider carb, this guys has big meaty claws, carries a giant Cut Ribbed Ark shells as shield and Auger shell as spear.

Grass Starter (Dodo bird-->Andalgalornis steulleti-->Cockatirce)
Sword (Grass-->Grass-->Grass/Dragon)
>First stage: a dodo with a pooly sharpened beak, cannot properly cut, but doesn't mean it can't make any kind of damage, small feathers.
>Second stage: a bigger flightless bird, nimble in look, with long feathers that are grass blades, the beak works like a knife.
>Last stage: a cockatrice, few long feather and a long tail, both of them are sharp and used as swords the beak is still like a knife.