Value parameters
1- Denomination
2- Rarity
3- Metal scrap value
4- BTC scrap value
5- Utility , as a hardware wallet
New versions will lose simple denomination, but the hardware wallet value could be enhanced.
a) Durability, I see other products able to survive high temperatures with codes built into the metal.
b) A public address QR code to make balance checking easy.
c) A signed QR code balance sticker generated by Casascius, to place on the new coins later.
I believe the reason that the codes aren't etched into the metal is because common medical imaging equipment could read the code without removing the holo.
Regarding point c, can you elaborate? If you're suggesting a second public/private key holo that the purchaser could apply after the fact, I don't think that would ever be something that'd be offered.