You should be able to let the player customize numeric knobs and toggles, e.g: pity rates, shop price, pull price, feature toggles. Those should work out of the box because they’re global values/dicts that get re-read every time they’re used. Adding a button that, say, increases something by 1, should be easy to implement: https://www.renpy.org/doc/html/screen_actions.html#data-actions
So: textbutton “Hard Pity +1” action IncrementVariable(“PULL_HARD_PITY”, 1)
or:
textbutton “SSR Rate +1%” action IncrementDict(BASE_RATE, “ssr”, 1.0)
What wouldn’t work are things like adding/removing a whole rarity tier, character, card, or banner. Those are read once when the game launches, so the plugin would need tweaking to let the player change those in-gameplay. Off the top of my head, GACHA_CARDS/CARDS_BY_RARITY/CARDS_BY_CHARACTER/BANNER_FEATURED/ are some that wouldn’t work.