Custom license plates that say "His <car model>", "Her <model>" are fairly popular. If you just have to show off you affluence, what can do it better than a pair of "His Z4"/"Her Z4" in the driveway!
These plates, however, are confusing for the very person they are supposed to please: the owner. See,
if the husband drives his wife's car, he knows this is "Her Z4", no confusion here. But, when he is in his own one, he may think, "Ok, I am in his Z4... His? Hm, whose car is this? I gotta go find my car before _he_ comes and kicks my ass!"
So I think a shared "My <model>" plate should be issued along with His/Her ones, and when the car recognizes the guy (via face recognition for luxury models, or by RFID in the key for entry-level luxury), the plates are automatically changed to "My <model>".
By utilizing the wireless Internet, plates clash can be avoided -- i.e. when the wife already drives her car (that says "My ..."), the husband's car will offer him either to wait for his wife to arrive at the destination and release the "My ..." plates or go ahead and drive as is, turning on some relaxing music and saying regularly in a soft voice: "Dude, relax, this is _your_ car" and "Don't worry, _he_ won't come for you, this is _your_ car".