Put simply - this system would allow people to choose ONE class that they intend to stick with and gain an extra post-transcendent skill in their embraced skill which would add a new mechanic to the class.
This mechanic would be strong and potentially path-changing, but the caveat being that you may only have access to one of these mechanics per character, ever. This would be akin to a 'class specialization'. If it can be changed, it isn't something that can be done lightly. Maybe a quest would be needed to unlock this stuff in the first place.
Here's a few ideas for what these extra abilities could be:
Depthswalker: Aeonics - Timelapse
A master Depthswalker may mark a person at a point in time, and at a cost of 800 age, instantly revert the state of that person's afflictions and injuries back to the marked point. Using this ability when it would put you above the age cap will kill you outright.
Apostate: Apostasy - Exarch
Your summoned Baalzadeen becomes an Exarch of its kind, gaining a new appearance, 50% additional health, passive health regeneration and the ability to expend 20% of your health and 10% of its health to balancelessly use a daegger puncture attack every 15 seconds.
Monk: Telepathy - Stillness
You may cast the stillness of your mind into a protective barrier around you, briefly rendering you completely undetectable by any farsight-like abilities. This barrier lasts for 25 seconds or until you undertake an aggressive action, whichever happens first.
Priest: Healing - Absolution
At the cost of 30% angel power and a large mana cost, you may now raise an impenetrable shield around a target. No attacks may breach this shield, but every attempted hit against it will drain 5% angel power and place the person off balance for 1s. The shield lasts for 10 seconds or until the person shielded performs an aggressive action, or is removed if the angel involved runs out of power or is dismissed.
Blademaster: Shindo - Totality
At a nominal Shin cost, you may suffuse your form with extreme speed, all but eliminating room movement delay (delay per 20 rooms) and any balance cost associated with Shindo mobility skills (leap high, bounding, leap). In addition, you may dash while heavy in this state. Lasts for 30 seconds, but can only be performed once every 2 minutes.
Runewarden: Runelore - Runeaffinity
While active, this ability will allow you to sketch all runes with no ink cost, a nominal mana and willpower cost substituted in their stead. You may also sketch ground runes at double speed, but the skill will end the moment you perform an offensive action that isn't sketching a rune or holding a totem up. All rune effects will also tick twice as fast on both allies and enemies.
Paladin: Devotion - Bladefire
You may wreathe your blades in the holy fire of Templar's past, causing them to deal significant extra fire damage on hit. Targets suffering from an Inquisition will take double the bonus damage. In addition, any rites fervoured during bladefire will proc immediately on the next bladefire hit the first time they are created.
Infernal: Necromancy - Scourge
While putrified and with an infestation present in the room, you may draw the unholy maggots into your flesh and drive yourself into a terrible rage. Each strike in this state will give you large amounts of life essence. You may choose to spend life essence to bolster either your attack speed or your damage, but the frantic nature of your attacks will prevent venoms from applying with regularity during this state.
Alchemist: Formulation - Rubedo
Attempt to complete the vaunted final step of the Philosopher's Stone, Rubedo. You must be under the effects of Mayology for this to function, but you will find that for a brief, all-encompassing moment, you will become absolutely and utterly unstoppable by any mortal means. No binding will slow you, no damage shall harm you, no affliction shall affect you. Slaying ten mortals within the Mayology window with which you started the process will complete the reddening, and you will receive a completed Philosopher's Stone.
Sentinel: Woodlore - Packleader
The Wolf companion now becomes a permanent class loyal. It may be named and will assist you with attacking your foes, both denizen and adventurer. Summoning a wolf with Woodlore versus an adventurer will briefly massively increase the damage the pet does for a short period of time.
Druid: Reclamation - Merge
You may descend into the greenery of your plant network, becoming one with it. While in this state, you have direct control over your vines and where they choose to spread, and they will spread at a significantly increased rate. You may move your attention freely to any room in your network and target reclamation effects directly at one person, and the Wrath ability will not sacrifice any room but the one it is used from in this state. If your network is reduced to 1/3 of what it was when you started the merge, or the room from which you merged with is removed from your network, you will instantly die.
Jester: Pranks - Nimblefingers
You may now juggle up to 5 items at once, and such is your skill with juggling that any thrown items immediately return to you to be juggled once again. In addition, you may juggle Tarot cards, though they are destroyed after they are flung via juggling.
Magi: Artificing - Titan
Channel the power of your existing vibrations into your crystalline golem, and ascend its form to a towering beast. The titan will rampage at your command within the local area, pursuing targets and acting as a mobile tether for your vibrations until it is slain or until you are slain.
Occultist: Occultism - Window
You may use your knowledge of the ties that bind the planes of Creation to open a window to the Outer Cold. The act will render you immobile and completely unable to perform any action, but those who remain in the room with you after a certain duration will be hurled with you into the depths of the multiverse, and perish instantly and truly among the stars. The window will also actively pull people in from neighbouring rooms from a fair distance away, through mass or other blocking effects. So great is this demise that all will be forced to enter the Halls of Death immediately. You will gain massive amounts of karma for each target killed in this way.
Bard: Voicecraft - Finale
You may bring your harmonics to a thundering head, ending them instantly but applying a double-strength version of their buff to all mutual allies in the room for 15 seconds. Each kill you achieve during this timeframe will refresh the buff duration on all targets, regardless of where they are. The effect will fade if you perish.
Sylvan: Propagation - Greenblood
You may briefly cultivate all known species of herb upon your viridian form, and gain all of their benefits. In addition, curing in this state will not consume herbs of any kind, and moss, health and mana elixir potency will be doubled. Slaying a foe with heartseed during this state will allow you to retrieve the heartblossom, which may be consumed by anyone at a later date to instantly cure all afflictions and restore some health and mana.