There have been rumblings that, like London, you will eventually be able to use a contactless payment card to travel. That would probably be more generally useful than trying to merge the backends of the two systems (and they have quite a different ticketing philosophy).
London has accepted con tactless 'payment' cards on the buses since last year and it's rolling out right now onto the underground.
It's not really a replacement for Oyster, it's more of a way for 'casual' users to pay with out having to obtain a card.
TfL have issued warnings to people NOT to present a wallet full of cards to their gates from now on as an 'unexpected' card may be charged for the journey. This could be particularly annoying if you were expecting a 'free' trip 'cause you had reached the day cap, or the trip should have been validated against the pass on your Oyster and not charged.
This is the way of the future for fare payment I expect.
Cross acceptance of cards has more issues than just reconciliation, they need the encryption keys and file formats used for all systems they have 'reciprocal' arrangements with. Smart Card ticketing systems tend to place a lot of trust in the card to enable 'offline' operation, so they don't want those encryption keys wandering too far from home. Knowledge of the keys would allow certain types of fraud to be undertaken.
The credit card systems place far less trust in the card and instead focus more on back end systems detecting fraud.
BTW both Myki and Opal are using NXP Mifare Desfire. Gocard and the other earlier systems Mifare Classic. London is MiFare classic too, but I understand they are migrating to Desfire cards.
Most of the reader hardware can read just about anything - most of the newer cards available these days follow the NFC standard as does the reader hardware.
One annoying aspect of Opal is they haven't bothered to even attempt card discrimination, so you have to be careful to only present ONE NFC card to the terminal. So you can't present your wallet if you have a NFC credit card.
EMV credit cards can use a variety of card types - I've seen a Visa Prepaid implemented on a MiFare classic chip!. I suspect with the Comm Bank being involved in Opal, the acceptance of bank issued 'contactless payment cards' will come relatively quickly once the base system is in place.
Another annoying attribute of Desfire over Classic is significantly poorer read distance. I've already determined that an Opal terminal will read (and reject) my office building access card (MiFare Classic) from nearly 10cm from the reader plate, yet the MiFare Desfire Opal card has to be closer than 2cm from the SAME reader to get a valid read.
This is a known attribute of the technology, the manual for my building access system actually states that Desfire cards have shorter read range than a classic card - the system supports both.