Sounds very good, looking forward to this feature! It could for example be included in a way, that the player had to stay on a keyboard-key, to rewind the gameplay and as long, as he holds down this key, everything runs in reverse. If the player releases the key, he can continue playing the game from this point on.
A nice side-effect here, would be the following. There are some C64 emulators, like for example Hoxs, Denise or CCS64, that allows the user, to remap keyboard-keys to buttons on the controller. Then the player could rewind in the game, directly on his controller, offering excellent handling. And I also have another idea, regarding this point, that would make it possible, on a real C64 as well.
The C64 supports a second firebutton (Pot-X) and there are some joysticks available, that have such a "Fire2" button (e.g. the "ArcadeR" or the "Cheetah Annihilator" and also certain joystick-adapters). If the rewind function were possible not only on a keyboard-key, but additionally also with Fire2, then owners of the mentioned joysticks, could also rewind the game, directly on their controllers, even on a real C64.