If we look at the National Dex we will see that any of the pokemon that are in it can work as a Starter Pokemon.
We have 3 stage evolution, 2stage evolution, and split evolution so there's many to choose from, we may have to fix the trade evolutions.
If you play an Offical pokemon you get a 3 stage evolution, those who played a fanmade game May have gotten a 3 stage or a 2 stage depending on the game.
And will the recent Let's go games we had a starter in Pikachu and Eevee that can't evolve yet they work!
So look at the pokemon and see which one of them would you start the game with.