I’m wondering if, instead of deciding on one address format which has hardcoded support for shards/L2 chains addressing, it might not be getter to have optional address extensions.
So in the base 32 byte address, there could be a byte stating whether the address is followed by a second 32 byte extended address. If the byte is 0, there is no extended address information. If the byte is not 0, its value would specify the type of the address extension that follows in the next 32 bytes. Any address extension would be optional, but as you described providing it might be a way to optimize transfers.