Wanna know about how to submit shopify and what are the obstacles

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So i have condifident that i can build a shopify theme from scratch with all the medatory functionality . Now i wanna know from experts what are the keypoints i should follow to target and  build on a specific niche theme and successfully submit in envato elements market or shopify theme store. I will be very thankfull to you if you give me a good and successfull path to complete my dream journey. 

1.  How to research niche to build a theme so that it will help shopify marchent's ?
2.  what are the keypoints i should follow when i will build a theme ?
3.  which path i should follow to consume my time while i'm gonna build my project ?
4.  what are the requirements i should follow to submit a theme in shopify theme store &  envato market ?

Thank you so much for your answer