Just one question that I just thought about, is how would the Opal card work with the Airport link stations (International & Domestic) being that there is still a surcharge or "gate pass" fee to paid on top of the normal cityrail fares for utilising these stations??
Again, this is a relatively straightforward exception to handle. Assuming only single-use GatePasses (ignoring weekly GatePasses for now), all there needs to be is an add-on charge deducted on the spot when entering or exiting Domestic and International.
Weekly passes complicate matters, though, but you could run a check to see if an entry/exit at Domestic/International was made in the last 7 days; if so, deduct the difference between the weekly and single charge, and do not levy any extra charge for the remainder of that week.
(This is based on the weekly GatePass being only marginally more expensive than the single. To send my mother off at the airport, I purchased a weekly GatePass and proceeded to visit International several times while it was still valid, just for the hell of it.)