Five Diablo playable classes were announced to date, and they all represent different versions of D2R Items the previous games. Since each game has been different it is possible that Tyrael can be an important player in the fight in the battle, helping to balance the sides by offering players the opportunity to acquire angelic or demon-like powers in exchange for duties to Sanctuary. This kind of system could very easily build on the multi-tree development that was announced for the series.
It seems unlikely that Blizzard intends to turn Angels and Demons into classes, given that the series is centered around those who are caught between the two. However, as a mortal who is caught between Heaven and Hell, Tyrael sits in an unimaginable position in the entire Diablo the lore. It is likely that he will be playing a role of balance throughout the story, and maybe even an impartial voice, rather than taking sides as his former self. Even if he's forced to choose sides, Tyrael is now aware of what is important to mortals, and that is what he'll be doing to participate in the events of Diablo 4.
The way Blizzard speccifies the character of Diablo 4 is a blend of the strategy of the studio for Diablo 2 and Diablo 3. As the executive producer and director of the Diablo franchise Rod Fergusson put it, "Diablo 2 was like locking yourself in. You had the ability to respec every time you faced a challenge. But with D3 you kind of changed your build like you would diablo II resurrected items change your clothes. Everything was gear-based as opposed to skill-based."Fergusson added, "I think the fact that we have skills on the equipment [in Diablo 4] is really nice for experimentation -- as a sorceress I [might] get Blizzard [on a pair of boots] three levels before I should and I can try Blizzard to see if I actually want it."