I'd also like to see AirTag technology built into Apple's portable devices. It won't happen, but with a specially designed battery capability to keep it working after the devices battery dies would be useful.
I think some of that is already built in. I left my iPhone in a cab a while back and in a foreign country which made immediate reclaiming a bit more difficult. Though the phone eventually shut down, it did so with still enough juice to do a periodic ping to the Find My service which helped me track it down. It wasn't simply showing me the last location as I could track its movements. This was on iOS 17 so maybe that shut-down-but-still-keep-juice-for-Find-My is relatively new. I can't find specific documentation on that immediately: all Google searches are returning people claiming that Find My is draining their battery.