My favorite class in D2 was the Druid. I love the shapeshifting and whole lore behind him. I know a lot of people trash on the Druid so don't waste your time here doing that. Im proposing the first expansion class to be called the Hjghlander. You can keep the same sort of lore pretty much as the Druid, but change up his summoning.skills.into something else to stay away from the WD and elemental away from the Wizard. Anyways love to here feedback on skills and lore backing this, keep in mind it does need to be unique and not like other classes.
There's definitely room for expansion characters similar to yours because the five D3 heroes don't touch all the archetypes. Obviously, something like an Engineer wouldn't fit D3's style too well, but there's many more avenues beyond that. Shapeshifting and the Druid's archetype of "nature-attuned guy" are untouched, as well as other mechanics such as Spirit summons, manipulation of bone, earth, weathery things, and metal (think AvatarTLA metalbender or Fullmetal Alchemist). There's lots of fictional fuel for a new class or two, and definitely room to add extra skills to existing classes in future expansions.
Shapeshifting is awesome as a power, because you can pretty much base an entire class just off of it. Don't just think Bear/Wolf, but as others have mentioned Demon, demon-animal mix, some sort of Reaper figure, or just "monster." I'm thinking Spirit Reaper sort of thing, able to shapeshift into something, controls bone, blood, can summon offensive/defensive spirits. Really there's just too many directions you can go.
I like the idea of a highlander a lot, I think it would be a good fit for the Diablo theme. I don't think he'd be "attuned with nature" or anything like that, I'd see him as mainly a brawler with some elements to take him away from pure melee, kind of like a civilised barbarian in lore and playstyle.
See the only thing is on the half demon thing is, its too much like the Demon Hunter, remember they have to be human, raising the dead and trying to recreate a new version of the Necro is a problem to because theres the Witch Doctor. The only problem I have with the Highlander class is coming up with its own distinct skills. In my opinion I think they should focus on one expansion character and leave the rest of the second expansion time for a quality 5th act, but then again its Blizzard I'm sure they'll surprise me.