I think one obstacle to interface with a 2-wire system is I am not sure if there is any open standard for it — most companies use proprietary systems and they don’t publish any details on the inner workings of their systems. This would either require some reverse engineering (which could be very difficult) or have a way to interface with their controllers, but this sounds more like a DIY / Hack project than an elegant solution.
It’s also possible to develop our own 2-wire system that uses open standard, but it’s likely not worth the cost — each valve adapter probably needs to have a microcontroller and power circuitry to convert 24VAC to 5VDC, as is the cost is going to be quite expensive.
Regarding @tmittelstaedt’s point 3: I did recently come across U.S.Solid’s motorized ball valve, 24V AC/DC 2-wire auto return type — it’s quite interesting, it has a super capacitor inside so when the power is lost, the charge in the super capacitor will be able to drive the motor back to closed position. As a result, it does shut off the water supply when power is lost. It’s completely compatible with any 24VAC sprinkler controller, so an interesting solution to keep in mind if motorized ball valve is required.