It kind of makes sense. Not a hollow-shinigami, but not a shinigami-hollow either. A creature that is equal parts of each.
But what would it look like? Because of course such a perfect hybrid would have to look really cool and suave, but it'd also have to appear equal parts hollow and humanoid. Which could open up some cool possibilities, I suppose...
About the whole '3rd release' thing. I think that if there is a possibility for 3rd level release, anyone can access it. It's just that, however long it takes to get to bankai from shikai, it takes just as long if not longer to get to the next. So while anyone technically could develop it, it's safe to think nobody, or only a very few people, ever have.
BUT. The hybrid's releases altogether would definately be interesting. See, shinigami releases are all 'sword', offense-based. And hollow 'releases' are 'shield', defense-based. For an example of what a hollow release would be, think of Grand Fisher (hollow mode). His 'release' would be the whole body-switch thing. It doesn't do anything to physically hurt his opponent, but it makes it harder for him himself to be hurt. Arrancar, from what we've seen, have releases that are still very defensive but can also attack. So a hybrid release would have to strike a perfect balance, becoming a 'sword-and-shield' model. So while they might not be able to do a 'new level' release any better than anyone else, their original releases would be a lot more all-encompassing and thus more powerful. Complete offense, complete defense. Which is why creating these perfect hybrids is such an important goal to everyone; they'd be pretty much invincible in every way imaginable.
Why So Green And Lonely? We Are Accidents Waiting To Happen.