There are no skill trees any more. Skills are still tiered by when you can buy them (essentially a level requirement) but there aren't any trees, it's one big pool.
Before this game even reaches a beta state there will be hundreds of people playing it internally, and most likely there will be builds that are developed during that time. But the game is specifically being designed in a way that skill progression is as much of a choice as possible. There will always be builds determined to be 'the best' as long as there are choices and options. Making every skill and every pathway perfectly equal would mean watering down skills, making bland options, and overall homogenizing the classes and skill abilities. So it's a trick of balancing unique and interesting and fun skills while also ensuring that no matter which ones you pick you're not totally gimped.
I also want to point out that skill runes can have a fairly dramatic impact on how skills function (and even look) so builds are further compounded not just by the skills you choose, and itemization and other stats that improve those skills, but skill runes that can potentially change their very behavior. And those changes can range from small things like adding a chance to stun, to very extreme things like causing an AoE knockback, and any creatures knocked back repeat that AoE knockback from where they land, and so on and so forth. So there's quite a bit of room for experimentation if not build potential within all of that.