dont go balls out in one tree becoming super dps at the cost of support and defensive abilities.
the best chars are balanced
for example i am a mage-> necro warlock chloro i get a skelie pet and do tons of damage but can also heal and have stuns/fear/buffs.
I'm pretty much unkillable in pve because i have many options for survivability even when i aggro like 10 guys.
In groups I am super useful for when tank derps and pulls shitton of stuff i can help heal so we dont wipe and i can crowd control as well, when they pull good i just do shittons of damage.
So really just pick whatever you want but make sure you can do at least two roles (tank, heal, damage, support)