My reason for this is because players like me want to create adventure maps based on other forms of pokemon. take for example shadow lugia. you would have to waste your shiny texture for a shadow lugia, which i can agree is a fair trade but what i would still like to have more options. also this doesn't just have to be for shadow pokemon, this could be used for made up types of pokemon such as delta species or used for alolan forms, and other various types. that and not sure alot of people would be too happy to waste shinies just to get more forms for each pokemon.
This definitely isn't something that's high on priority but rather just to give people more options with what they want to do.
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