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Personally, I think the idea of creating a fan comic that you're excited about to kind of get in the groove is fine, but it seems to me that you couldn't post it on Tapastic without breaking the rules. I'm pretty sure that if it's Actually The Digimon World, even if you changed up character designs, it'd still be their IP? Though I do like @Kaykedrawsthings's idea, if that's something you think you could get into!
I'll also say, I think it's okay to learn as you go for your first original story, too; especially if you make your first story a shorter story!