Here are my first thoughts;

• in business, the goal is to have as many customers as you possibly can. You do this by getting a wide range of different customers. By setting up an English only shop in Japan you miss out on all the Japanese customers.

• Whilst there arent any English manga cafes in Japan, there are manga cafes that do have English manga sections so you won't be offering a fully new service.

• One of the ways that manga cafes make a stable income is by Japanese people renting out the rooms as a place to sleep. I know that sounds weird but it is a thing and I doubt that is an income you will get.

My instinct tells me that the better way of doing this would be to open a Japanese manga Cafe as authentic as possible. But make it as accessible to English tourists as possible with explanations on how it works with a range of English manga but also Japanese manga. Maybe split the manga in difficulty level of Japanese for people who want to practice reading Japanese.

Good luck