as someone who wants to dip their toes into all 3 aspects of what you're attempting (Cannon Mons only, Fakemon only, mixed bag) I would say it'd honestly be better to begin by establishing large general sections of your world and then "zoom in" on those bits. for the creatures themself, the option is ultimately yours, just be mindful that some awesome story ideas won't be possible in a "Official Mons Only" game. If you want to build upon the pre-established lore of the pokemon world we're familiar with use cannon pokemon. if you want new lore, introduce a legend or two, a few regionals/megas/gigantamax/(insert other gimmick here) of your choosing and go ham, pokemon works on multiverse theory anyways. if you want a world truly foreign and open to exploration with each new map being mystical and unseen, a fakemon game is your best choice.
To truly get yourself in the door, I highly recommend a simple casual adventure in a region of your own design. 8+ gyms max if you wanna get spicy. The simplest way to answer this question however is to do this.
1) Sit at your computer
2) Open up a text editor of your choice
3) Type in "What do I want in my game?"
4) Make a small list.
5) Profit
once you are in the door, then you can let it spiral outward from there once you have friends onboard to help.
Thank you for coming to my Ted Talk
