In general, with the 2G ZR-1's the parts which are specific to that model are the engine (obviously), LT5-related electronics, the transmission, the windshield, bodywork aft of the B-pillar, the rear wheels, in some years, the exhaust and certain trim items.
I'm not sure I agree that ZR-1 specific parts are "generally available" but I suppose part of my hesitation to agree might be how one defines the term: "generally available". As I see it, the availability of ZR-1-specific parts is spotty. For example...try finding a windshield, some of the rear body pieces, an LT5 crankshaft or an oil pan.
Another problem, and one which affects the owners of all C4s, not just ZR-1s, is the availability of replacement parts for various pieces of the car's electronics. For example, try and find a new lat sensor for the 90-up ABS or try and find a new CCM for the late C4s. Not gonna happen because GM no longer services those parts, the reman industry doesn't service them and the reproduction industry doesn't have the capital investment available to make the parts. Also, in the case of parts like the ABS lat sensors, no aftermarket reproducer is going to take on the liability of making brake parts.
Unfortunately, the lack of parts like lat sensors render the ABS inoperative and if the car doesn't have a CCM it may not even run.
This is not to say that C4s are no desirable as collector cars but it is to say that as C4s continue to age, those in the hobby who enjoy owning them will have more significant challenges to meet than have those who own C1 through C3.
As for ZR-1s becoming more desirable collector cars from a financial perspective?
If our time horizon is ten years, I'd say the national economy in the long term needs to improve and have less uncertainty before ZR-1s are going to appreciate much. In general, the 90s and 91s, because they were built in much larger quantities than the 92-95s will be the last to become "valuable". Now, there will be a few exceptions to this, ie: cars with unusual histories, cars owned by celebrities, cars with rare RPOs and etc.
The 92-95 cars are likely to appreciate more rapidly but, I think it's going to be many years before prices really take off.