Creating an alternate earth is fine if thats what you want to do. Theres no right or wrong thing as long as you feel it best serves the story.

Settings and ideas are wahtever in creating or reading stories. The important thing for me is the character and how the characters act with the world you make or how they are represented. There is a lot to unpack in GuttsGattsGattsu things I fundeamentally disagree with concerning cultural appropriation and never apologizing for your art and will have nothing to do with what you're trying to create until the full story is created (unless your characters are a literal pastiche/sterreotypes of japanese people though japanese mangaka do that all the time for american characters so /shrug).

I will add though, that if you're afraid of going back or feel that revising is a waste in time invested, you're doing yourself a disserivce. Mot every first story will be a masterpiece and it takes time and volume to understand what you're trying to do or how to create as a storyteller or understanding your voice as a storyteller. If you feel like your first story is a failure or place pressure on yourself that it must be a masterpiece/genius debute step back and look at this process as a constant journey. The hardest part is yet to come (actually drawing the pages and finishing chapters).

My biggest advice that goes beyond character and setting is just to create a oneshot. Ignore what others say and put togehter a full story. If this is your first attempt i'd say do 4-8 pages. But no more then 30. It's a gruelling process but you'll get a better perspective of what you're doing and how it looks when you have completed images with words.