The passive skill tree as it is now is very limiting and feels very lazy. If Diablo 4 is looking for a huge way to allow people customize their characters they could share skill trees. This would not affect what skills the character could use, but just how they will use them. Lets say I wanted a Barbarian who uses double throw, but I want to throw lightning axes. I could have access to a passive tree that increases lighting damage. There could even be a node in that tree that turns all of my damage into lighting. Or have a ranged node that splits your projectile attacks so you can throw a bunch of lighting axes. This could lead to me making a sorc that uses lighting meteors or lighting orbs. Or, if I wanted a Sorc that can tank and use more close up attacks, I have access to a tree that can boost defense and resistances. Maybe a node that gives double resistances when your resource generator is below half. (This would have zero affect on the available skills, but would lead to a ton of ways to play that character)
A passive tree could very well be linked with attributes like angelic power etc.