Guesses
Froakie as the Water starter. Chose non-Sinnoh starters in Legends: Arceus, sure. Do you think they will deny a popular Pokemon in their native region? Could be a boss still, of course.
Snivy for the Grass starter. Originally thought Chikorita as a cute option. Realized a good Serperior regional/mega design around the fleur-de-lis. Suits the "Regal Pokemon" well. Adds this line from Wikipedia:
Models this game's setting after mid-to-late 1800's Paris.
Litten for the Fire starter. Says this half out of popularity, half out of unleashing Mega Incineroar upon the world.
Predicts one step further with regional or mega typings.
- Greninja: Water/Poison. Seems reasonable for a ninja to use poison. Probably uses the tongue as a weapon instead of a scarf.
- Serperior: Grass/Steel. Places the fleur-de-lis on banners and metallic things (like crowns). Shrugs.
- Incineroar: Fire/Fairy. Plays the role of the heel normally. Reverses that. Sticks in a new type combination too. Loves doing that for regional forms.