But isn't there a literal in-game Boon from the DMG that grants an additional 9th level spell slot? Wouldn't that satisfy your condition for it being in-game and not meta-game since you consider rerolling a d20 to be an in-gams thing and not a meta-game thing?
I mean, its not exactly the slippery slope you're making it out to be. Though if a player were to ask to roll a d30 instead of a d20...I might allow it for a single roll provided they have a physical d30 on them. XD If only because that would be a very interesting dice to see.