According to this GDC video, selling new levels is not a good idea because only a small fraction of the users ever finish the free linear content. Average paying player makes their first purchase on the 22nd play session, and it's best to keep reminding the player often about the possibility of in-game purchases. In other words, the whole game is a store of sorts where reward/annoyance is carefully managed. My original plan was to sell episodes, but now it's clear that it's best to sell weapons and upgrades, so I started on my in-game purchase experience design.
According to this website, red, orange and yellow are the best colors for a storefront. Fateless world is largely orange and yellow as it is, so this made it only more convenient to make the storefronts orange and yellow. They will sell weapons, other upgrades and possibly, even tanks. Tanks are huge. I wonder what that would even look like. Maybe I should look into car dealership design?