As everybody is saying, the idea is a really good take on the theme! I agree that it could be a bit more difficult, or more fast-paced anyway. Maybe you could also have some form of danger, like moving cars for example, that you would have to avoid in the 'real' world?
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Thank you for the suggestion! Something in the "real" world too would be cool too. My initial thought was to have the ability to swap between worlds when you didn't have a package, but make the real world navigation slower. However, if it was more difficult to cross the street or something and forcing the player to almost obey traffic rules (that they could ignore in the Underworld), that might work too.