You don't need such things for lower difficulties. Some of those toggle ability upgrades are pretty damn broken if I'm honest. Below are some obvious examples I immediately thought of.
Mages:
winters grasp - winter ruins: amazing damage for what it is
fire mine - flamming array: triple fire mine spam with no prime time, what can possibly be better than this?
immolate: the upgrade removes any CD on this spell, and it's now your new basic attacks
barrier: an upgrade to effectively make it last a few seconds longer, useful stuff
spirit mark: the new upgrade makes you a machine gun turret , great synergy with static cage
Rogues:
Poison cloud: effectively party wide god mode. Let your characters dodge any 3 attacks and immune to dot damage while you have at least 1 stack of elusives
Throwing blades - precision targeting: over exaggerated damage bonus and sunder effect ever since the devs "fixed" it
leap shot: free stealth
elemental mines: bigger mines which applies statues effects are awesome, much less of a stamina drain monster too
hidden blades: skill is OP already, and making it AOE just made it even worse
all the tempests flasks: those flasks are what makes tempests tempests.
Warriors:
combat roll: ridiculous damage with sub zero CD and stamina cost
horn of valor: party wide 10% guard on hit is plainly ridiculous
livid: unlimited duration for dps warriors
Wombo combo for templar, dragon rage and devour, walking fortress and To The Death have always been OP right from the start.