-Tyrael: Ok a lot of people believe he's good and I'm sort of inclined to agree, he is definately anti-hell and somewhat of a renegade angel now, I sort of doubt he will be a boss in D3 but I could see the plot twisting and him being a major enemy in an expansion maybe.
-Azmodeon and Belial: We know these two Lessers are still alive and kicking, in fact they were the ones who orginally conspired to kick the Prime Evils out of hell so they could get back to fight the war with heaven. Unfortunately they decided to betray each other and Hell became stuck in another civil war instead. So apparantly they hate each other, either one seems like a very possible candidate for an Act boss.
-The heroes of Diablo 2: May seem a bit of a stretch but they were said to have all gone insane (besides the Barbarian apparantly) shortly after the events of LoD. The insanity issue seems to be a common side effect of mortals doing battle with or being in close proximity of demons. Therefore I think while they may not be Act bosses, maybe one or two of them could show up as enemies at some point after being driven insane by the armies of hell. Also the fact that in Diablo 2 you actually fight against all 3 of the original Diablo heroes (the warrior who becomes Diablo and the sorcerer who becomes The Summoner, the rogue who becomes Bloodraven) is a giveaway that Blizzard is willing to use old heroes as enemies.
-The Angels of Heaven: I would say there will definately be angels in Diablo 3, probably showing up a little later in the game, say act 3 or 4. I also believe we will be doing battle against them. This allows for a lot of different possibilities of angels to fight, the main contenders being the Angris Council (basically the Prime Evils counterparts), among whom Tyrael is/was a member. The two that would most likely be up for a big showdown would be the archangel Imperius who was most vocal about simply destroying Sanctuary and who was backed by the archangel Mathael. Auriel and Itherael were Pro-Sanctuary and opposed to destroying it so its more unlikely those two will show up as enemies but who knows. Anyways, if the Heavenly Host is invading Sanctuary you can bet a few of the Angris Council will be among them.
-Inarius the rebellious angel who created Sanctuary with Lilith and was a major player in its history but who was basically banished to hell at Mephisto's behest. Currently whereabouts unknown but you can be sure he would have an interest in how the fate of Sanctuary turns out, I could see him being freed (perhaps by another desperate renegade angel?) and having an influence in events. Update: He's said to have been changed into a Balrog demon and someone mentioned he was banished to some other plane. So perhaps his chances of showing up in D3 are quite small.
-Lilith the Lesser Demon and Queen of the Succubi, Inarius lover. She coud potentially show up though I'm not sure about her whereabouts, as I'm not certain if the Pandemonium Event in Diablo 2 is really canon material or not. My guess is no, but that doesn't rule out her entering Sanctuary with the rest of the demons. And she definately would have an interest in Sanctuary just as Inarius, having been a huge player in its history. Anyone have ideas on this? Update: Someone said Lilith has also been banished to some other realm or plane. I'm not a 100% convinced however since we know that Blizz brought her back for the Diablo 2 Pandemonium event, she is actually the one who summons Diablo to help bring back the other Prime Evils as well as Duriel and Andariel. While I'm not sure if the Pandemonium event is supposed to be part of the lore, I would say its safe to assume that none of the above demons or Lilith herself should be counted out yet.
-Horazon and Bartuc the Warlord of Blood: It's a possibility these two brothers who ended up hating each other and who both messed with demons and summoning could show up in the game as well, doubtfully act bosses but more likely smaller players. We've already killed a Warlord of Blood in Diablo 1 and the last unique before Baal in LoD is named Bartuc as well so maybe he'll be making a third appearance.
-The Butcher: I seems quite likely we could see this guy again seeing as how Leoric is back as well, and the Butcher is a real fan favorite. Maybe a Butcher, Smith, Hephaesto tri-fecta?
-Nephalem: I've hit the character limit in this post so I'll be removing the Nephalem as I can't really find any specific character who would realistically become a boss in d3.