What has surprised me is how few Oyster card designs there have been. (I'm not counting Staff/Freedom passes etc; there are dozens of them.)
There is the standard blue one, the Barclaycard one, the Royal wedding one and now the Queen one. I have heard there is going to be an Olympic one.

Do you remember phone cards? Loads of different types... There was a large tulip-style collecting frenzy around them, years ago.
All finished now, of course. But BT and Mercury made some money out of them and I'm surprised Oyster has not tried to do so as well.