In a game with possible RPG elements, would you rather have classes, classes with separate trees, or an open tree to choose?

Game Info

Name - TBD (Turbulence?)

Style - 2D Top down Action/Shooter/RPG, multiplayer

By that I mean you directly control your character via keyboard, and each class has many different abilities to complete it's objective(either kill the enemies or a certain goal, etc) By abilities I mean something like WoW abilities. One class is going to be focused on DPS, so it can shoot the enemy with various shots with different effects. Once class will have more AoE attacks. So most things are ranged attacks with various effects, then they also have other utility moves, but the focus is on combat.

So the action and shooter elements are clear. IT becomes a bit of an RPG because I want to have various classes and items.

So back to my question

In a game with possible RPG elements, would you rather have classes, classes with separate trees, or an open tree to choose?

Classes

Example - Team Fortress 2

There are the 9 preset classes(if I remember correctly). In each you are capable of changing their utility by changing their weapons. I'm not saying I'm making a shooter, this just fits the example - each class has a role and some wiggle room.

Classes with Separate Trees

Example - World of Warcraft

Each class has a play style to a degree, but it can be changed drastically via the talent tree

Open Tree

Example – No idea

There is one giant tree to pick stuff from. Some are abilities, others not. Can upgrade the ability by going down tree. It's as if the tree's for each class in WoW just all got mashed into one

Right now I prefer an open tree. It might be hard to balance but I think it could be much more fun