Would be nice to have the ability to generate additional card numbers from within the app, which could either be set for one-time use or multiple uses. This would be handy for paying for something where you don’t totally trust the website; Or if your card details were part of a data breach because some company you use doesn’t have strong security, you can just cancel that specific number and no other transactions are effected, then you wouldn’t need to have a new card issued.
Similar to the above, but a cool benefit for black and metal customers could be given them the ability to customise parts of the card number when they sign-up. I’m sure the first 4 digits are fixed, but I think the next 8 are somewhat unique to the card so may be able to be customised. That said I know there’s some kind of algorithmic generation of card numbers that goes on so maybe this isn’t possible, but if it was, it would be a cool feature.