I'm not quite keen on the half vampire/half demon idea. The whole story on Diablo is about Hell and Heaven against each other, and humans in the middle. That is why so far, all our characters had all been humans.
A great character idea would be the ability to choose a side. The side of Light (Heaven) or Darkness (Hell). I'd like to believe that if you chose the side of the Light, you'd be able to acquire Guardian Angel (like, an angelic merc). If you chose the side of Darkness, you'd be able to have a Demonic Mercenary. Now, of course, there must be a special attribute/quest met before having this. Perhaps, on the very last act.
It would also be nice if they could be carried through the modes (Normal/Nightmare/Hell). And that, you'd be able to pick your partner before you even start the game. (Example: once you're in the character screen, you'd be picking which character, and then, you'd be able to pick the corresponding angel/demon for that character; and of course, the side of Light and Darkness would be in two different tabs). Just a thought.
i love the idea that you can ride horses. You can buy them like mercs, and they automatically kick enemies, you can also make them run, jump(to avoid enemies).
Oh, I most definitely agree. But, I think a good quest would be great to trigger this special function.
I think it was in Kurast, where you will be unable to hire the Iron Wolves Clan unless you complete a mission first (but then, after you complete that, Asheara gives you a merc for free). I think that's how it should be for special cases like these.