Nope. The architectures are just way too different for it to even be feasible. The Atmega line is missing too many features this would need, like memory-mapped

Nope. The architectures are just way too different for it to even be feasible. The Atmega line is missing too many features this would need, like memory-mapped IO to be even vaguely feasible as a replacement. You might be able to wedge a low-end ARM microcontroller in there, but it would very much be wedging, and you'd run into way more problems than it's worth.