Add a passive to each class tree that only activates with another same-class tree.

CameraBeardThePirate

Title gore, I know, but the character limit really didn't do me favors.

In order to promote pure-classing, each class tree should have an extra passive that only activates when another line from the same base class is present. This idea has been tossed around a bit, but I haven't really seen it expanded upon much. Let's take an example from something like Necro (noting that this will mostly be from a PvP perspective and is entirely a proof of concept and not likely balanced):

Necromancer gains a few crucial things from subclassing. Namely, a better stun, easier/better access to buffs/debuffs like Breach or better recovery, and another strong Burst to line up with Blastbones. So what could we do to entice them to Pureclass?

Gravelord Passive - Empowering Necrosis: Your mastery over body and bone causes your necromantic power to strengthen your own mortal vessel. Casting a Bone Tyrant ability empowers your next Gravelord ability to apply Minor Intellect and Endurance for 10 seconds. This effect can occur once every 10 seconds.

Bone Tyrant Passive - Defiant Unlife: Your mastery over unlife makes you more resistant to that which would do you harm. Casting a Living Death ability causes your next Bone Tyrant ability to grant immunity to snares and immobilizations for 4 seconds.

Living Death Passive - Stunning Souls: Your mastery over the manipulation of soul energy stupefies those that witness it. When you consume a corpse while having the Grave Lord tree active, nearby enemies are stunned for 4 seconds and take 5000 Disease damage. This effect can occur once every 8 seconds.

Each class tree should have a special passive that only unlocks when another class tree for the same class is present on the character. These should be powerful utility effects or take the place of buffs or other skills that you otherwise don't have in your class kit in order to incentivise keeping a pure class. As is now, subclasses so vastly outperform pure classes simply because you can cover a class' weaknesses - for Necro, you can get better movement, buff coverage/debuff coverage, and an on demand stun/burst by subclassing. Adding these into the class kit, but only while pure-classing, helps incentivise remaining as a pure class while not pushing the upper limit of power too high.

No, I'm not saying these examples are perfectly balanced or even necessary, but the "While you have X class line, Y class line does this" formula could easily be applied to the entire class lineup.

Erickson9610

This concept should also be used to promote unique Subclassing combos. For instance, combining Winter's Embrace with Grave Lord could further boost the Frost Damage of Boneyard while a Winter's Embrace ability is active on you.

There are a lot of interesting combinations that could be expanded upon, rather than just sticking with the skill lines that belong to the same class. It's incredibly boring and predictable trying to make combinations out of the skill lines which already come bundled together at character creation.