For what it's worth, I had an idea for a prestige class that was only unlocked after you beat the game (similar to the Scion in Path of Exile) that was kind of like this idea. My idea was that since Leah proves that humans can have half-demon children who aren't necessarily evil, you'd have a 'half-breed' class, who is a hybrid Strength and Intelligence class. The class would play kind of like Lee Sin from League of Legends (for those unfamiliar, Lee Sin's abilities all have a single activation, and then a second activation that does something different if the first activation succeeded). For this class, the Half-Breed would start as an intelligence class, similar to the Wizard except with more of a demonic feel to the skills (probably like a playable Maghda Cultist, with the hellfire bolts and summoning demons as pets/tanks).
The Half-Breed's resource would be Instability, a resource that was previously made for the Wizard before being scrapped. The old instability was like an overheating meter: it started empty, and gave you a buff for keeping it low. As the globe filled, the buff started to go away, and when it hit the top, you got a large debuff and had to wait a few seconds without casting spells so that the meter would drain and the debuff would go away. According to the developers, this was cut because people just kept playing regardless of the buff or debuff; they would just tailor their play to levels where they could steamroll the content regardless of buff/debuff, similar to what players do now with MP levels (and, probably, will do with Torment levels).
The Half-Breed's Instability, on the other hand, would be much harder to ignore. As I said earlier, the Half-Breed starts as an Intelligence class, but it doesn't stay as one. Every Half-Breed spell, with a few exceptions, raises Instability, and when the globe hits full, the Half-Breed transforms into a massive, hulking demon. At this point, the class changes into a Strength class (so the human part uses int, and the demon part uses strength), and all of the skills turn into Demon-counterpart skills, which drain Instability instead of adding it, again with a few exceptions. Thus, there would be three core ways to play the character, regardless of builds and special legendary/set affixes: Int-based human (take the few non-Instability skills and play carefully), Strength-based Demon (take the highest Instability-givers and play as a Demon as much as possible) and true hybrid (take both strength and int gear and try to switch forms when one form stop being optimal for a given situation).
As I said, the class would obviously have to be a Prestige class, as its mechanics are much, much more complicated than any of the other characters. Also, I don't believe that the D3 devs will do this idea, because even without taking the mechanics into consideration, they have stated that they want all of the playable characters to be noticeably human (which I believe is a mistake, especially after how they played up Leah), and this character is by definition not human (all the time).
As for other ideas, I like the idea of having an angel class, but there's no reason to have a reaper class, since the reapers in Act V are just angels that have turned away from Heaven and are following Malthael. You could still have an angel class with a few reaper-like skills, but there's no reason to have a full reaper class. As for an angel class, I would probably say that it should be either Int-based (holy magic) or Dex-based (fast combat using flight as mobility). In terms of strength, the only angels that give me a strength-feel are Tyrael and Imperius, and while they are the most obviously recognizable angels, they are probably not indicative of their race. That said, strength-base could work too, it's just not what I see when I think 'D3 angel.' Of the three styles, I'd probably lean towards Dex-based, as the Int-based one would be a holy caster, and we basically already have that in the Crusader.
