It doesn't make a lot of sense to re-issue a multi-function keyboard that adequately covered maybe one or two functions, but handled others with a significant amount of compromise. Yes, the ARP Quadra looks the part, but what it really needs is an Arduino update (along the lines of the KiwiTechnics stuff) with sample & hold functionality that gives you full slider recall, MIDI, etc.
We forget quite often, unfortunately, that the available microprocessors of thirty-five, forty years ago were expensive and complicated to program, and required enough support silicon to make products such as the Prophet 5 much more pricey than their modern equivalents. IMHO there is enough of a market for re-casting existing, already-manufactured synths with updated capabilities / new features, using hobbyist hardware that can be easily maintained.