It's not a bad idea (I once considered something based on parts compatibility between various Joes, such was what arms work with what torsos, but the sheer enormity of it was overwhelming and I pretty much ran screaming from the idea after a couple of hours) but as Morian Miner pointed out, there's such an...inconsistency among neck ball sizes it would create problems all it's own, especially if you aren't exactly sure which figure you're holding (for example, if you got a bunch of them, or parts of them, in a lot and have no handy reference.) Star Wars figures are pretty notorious for the same figure having multiple sizes of neck ball across the versions, for example. And again, using what Morian Miner said, sometimes they fit the ball but you wind up with either no-neck, or a giraffe-neck look because it sits too high.
It's doable, and would even be helpful, but you (or anyone who wanted to undertake it) would have quite a job on their hands, and you'd want to keep it updated as new versions of figures were released.