yo okay
making a game is great, and it's a skill that everyone should dabble with at some point
that said, I'd do two things:
1. don't make a spinoff of something else, it won't be congruous by comparison and the fundamental idea of the game won't come through the way you think it will. also, coming up with original content is arguably the hardest job as a creative designer, and practice in that alone is vital.
2. don't ask the forums for really big general advice on what the game should be. your job as the designer is to... design the landscape of the player's experience. the only way you'll be able to get practice in terms of knowing what makes a good game is to make it, refine what you've made, ad nauseam.
good luck!
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