[Hall of Famer]

Yeah just like Kyttias said, features will make its way to the core if its highly demanded by many users, and that its customizable to suit each different user's needs. If you request a highly specific feature that only your site will use, its unlikely it will be accepted into the main script.

The reason why we dont have rarity feature in the core is that its conflicting with the current adoptable conditions system. For the same reason users cannot sell pets between each other, the adoptable shops only sell pets from the system itself, as it will render trade system useless. If users are allowed to sell pets between each other, then trade system needs to be deprecated and removed.

Its a hard decision that we have to consider thoroughly. This is why a feature like rarity is better suited to be a Mod/Plugin, as you can install it and disable Mysidia's built-in adoptables condition system, therefore they wont interfere with each other.

Regarding item functions, yes in next version there will be a documentation on how to do this and how to create custom item functions. I may even find a way to make it possible to add item functions through ACP, although it will be tricky and requires code generation. But of course, it will be much more convenient than what you can do with Mys v1.3.4 right now.