Hmmm first thoughts are like gems you get by completing the different worlds, that you can use to upgrade the shop and get more chance to get better options, not sure how complicated that would be though :P
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oh wow, really like that idea, was thinking of a boss reward shop after boss fights with more permanent style upgrades but I'm liking this build the shop up idea even better, can even make each world upgrade the shop in a diffrent way, maybe even give the players a choice bewteen two worlds when they move on so theres less rng. really loving this idea!