i have an idea for the rebirth system, since diamonds are the rarest you could add something in the shop worth 100 of them (or a little less.) then it summons a boss or something that you can mine. and when you kill it, it rebirths you giving you like a luck boost or something cool like that.
Thanks for the suggestion, I do plan to have an overly expensive items in the game that increases the difficulty of the game, but maybe not a "boss", as we do not have monsters in this game. A mining challenge is a good idea. But for now, I plan to polish the existing game mode further before we move to the next mode. There are still quite a bit of contents that I'm planning to add.