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As much as I like the grass/normal simian, we've already had two grass apes in Sun and Moon, and I feel like the nondescript forest fox spirit thing is the next best grass design. I would have also assigned it a ghost typing, but we had a grass/ghost starter as well.
The fire serpent is cool as frick and opens up the opportunity for a cool typing we really never see. It also gives GF a chance to redeem their previous serpent starter with one that isn't the worst starter design to date.
Platypodes are cool and anyone who thinks otherwise needs to be cleansed. This also allows for a revival of the gender difference mechanic. The male platypus would have a Poison typing and access to moves like Poison Jab and Venoshock, a hidden ability like Poison Point or Corrosion, as well as a higher Attack stat. While the female would have a Fairy typing and access to moves like Misty Terrain and Play Rough, a hidden ability like Pixelate, and a higher Special Attack stat. Plus it's adorable as fuck.
Similar gender-based differences could be assigned to the other starters and several other Pokemon as well.
And with these starters, it sets up for a neat forest swamp region.