So if DAI had followed DA2's style of companion specialization, how would you have changed them? I'm really not good at balancing combat mechanics but I'll give it a go. (Note, I'm only using existing abilities from SP and MP and not creating new ones, as that would be a bigger challenge than I can take on)
I'm making my list, and really even something as small as changing the name of the ability to better suit the companion could have gone a long way to making the specialization seem customized.
Cassandra (Seeker)
Active abilities:
An Unquenchable Flame (Spell Purge): Through faith and will, you dispel all hostile magic from the area around you, dealing slight damage to the caster (mage, templar, or demon)
Seeker's Persistence (Walking Fortress): You may not be able to hold them off forever, but right now, nothing can touch you. You have complete immunity for a short time.
Spirit of Faith (Fade Cloak): You call upon the spirit of faith from your vigil to surround yourself with the magic of the Veil itself. You are briefly invulnerable and can pass through enemies unharmed.
Passive abilities:
Tranquil reassurance (Winter Stillness): By standing still, you enter into a meditative state that restores your mana at an enhanced rate and reduces all cooldown times.
Marvel at Perfection (Valiant): The seeker's faith strengthens her, mind and soul, for protecting innocents and punishing demons.
Religious fervor (Balanced Combat): Your next basic attack after successfully blocking gives bonus stamina.
What One Man Gains, Another has Lost (Resilience): You don't flinch, don't blink, and don't back down. Enemies that hit you with melee attacks are staggered by recoil.
Focus ability:
The Maker Shall be Her Beacon and Her Shield (Resurgence): You call on benign spirits to restore you and your allies for continuing the fight. All party members are healed to full health, including those who have fallen unconscious, and a glyph around you provides ongoing healing to the party for the spell's duration.
Iron Bull (Ben Hassrath Spy)
Active abilities:
Gaatlok Surprise (Ataashi's Breath): You unleash an elemental attack on all enemies that stand before you.
Ebost Issala (Crushing Leap): You leap into the fray, causing a burst of damage when you land.
Tal Vashoth Rage (Rage of the Avaar): You lose all composure and throw yourself into a wild rage that numbs you to pain and gives your strikes tremendous power. The more enemies nearby, the more damage you do and the greater your own damage resistance.
Passive abilities:
Those Things Ain't Natural (Champions of the Just): Your fervor inspires the entire party to fight harder against demons.
Meraad astaarit, meraad itwasit, aban aqun (From Knowledge, Strength): Whenever an enemy dies near you, your maximum health is increased.
Pashaara (What Is and Is Not): You deal extra damage to barriers.
Var-toh katashok, ebadim maraas issala toh (Hastened Demise): A wounded enemy is the perfect target. Your attacks are even deadlier against targets that are close to death already.
Focus ability:
Nehraa Qun (Rampage): You're an unstoppable fury of physical force while this ability is active. Your attacks are harder and faster, and you gain health with each strike.
Blackwall (Grey Warden Recruiter)
Active abilities:
Vigilance (Line in the Sand): You call upon the legacy of the greatest champions in history, defying enemies as you hold your position. This stops enemies from moving past you and enables you to block choke points.
Blood Fury (Ring of Pain): You mark part of the battlefield as yours. Enemies inside the ring take spirit damage while your own attacks hit harder, The more you are hurt, the stronger both effects are.
Blood Thirst (Devour): Blood is life. You rip into your enemy, doing damage based on how badly you're wounded and healing yourself.
Passive abilities:
Lonely Vigil (Biggest Threat): You make yourself the biggest target on the battlefield, the warrior no enemy can ignore.
In War, Victory (Adamant): You've trained hard, and you know how to make the most of whatever armor you're wearing.
You Will Guard Them and They Will Hate You for It (Bulwark): You stand all the stronger to finish the fight, gaining a bonus to your maximum guard.
In Death, Sacrifice (Unyielding): An attack the would bring you down instead leave you with a small amount of health, and you are immune to all damage for a short time.
Focus ability:
A Warden Never Gives Up (Counterstrike): You push yourself to the limit, gaining full guard and taunting all nearby enemies. While the ability is active, you automatically counter all melee attacks.
Varric (Self-appointed Chronicler)
Active abilities:
Say That Five Times Fast (Fast Beat): Increases movement and attack speed.
Always Leave Them Wanting More (Knockout Bomb): You hurl a grenade containing a powerful concoction that put nearby enemies to sleep. Sleeping enemies awaken after taking damage.
Have You Heard This One? (Battle of the Bands): Your stories increase the attack speed of your allies.
Passive abilities:
Gift of Gab (Tricks of the Trade): You help the team make the most of its abilities, increasing the damage and duration of all status effects the party applies.
It's Who You Know (Opportunity Knocks): When an ally critically hits, you take advantage of their success faster with reduced cooldown times.
Don't Interrupt (Can't Stop the Music): Your amazing performing skills occasionally cause enemy attacks to miss you completely.
Official Author for the Inquisition (Showmanship): Years of practice as a professional author have honed your talents.
Focus ability:
Everyone's a Critic (Hail of Arrows): You fire so quickly that enemies will swear there are at least two of you putting arrows into their ranks. While this ability is active, any archery ability you use is duplicated.
Sera (Jenny of the Red)
Active abilities:
Unforgiven Slight (Mark of Death): You mark your enemy, and every hit adds part of its power to the mark's damage. Trigger your mark when you are ready, and watch your enemy fall from injuries that were never even noticed. You can manually trigger the mark early for bonus damage.
Trick up my Sleeve (Hidden Blades): They'll swear you leapt from the shadows like the thrown blade and hit your target with a flurry of strikes from every angle... but its obvious that you were nowhere near them at the time.
Find my Friends (Fallback Plan): You place a thieves' lantern to mark a fallback location before heading into battle. When the fight gets ugly, you leap back to safety, as healthy as you were when you placed the marker.
Passive abilities:
Say That to my Face (On the Razor's Edge): You do more damage to enemies close to you.
Back Alley Brawler (Fury of the Storm): When your stamina is exhausted, you fight even harder, lashing out with fury to win the fight.
Watch Out for the Underdog (Disable): Critical Hits on an enemy reduces their damage for a short time.
Stick it to the Nobles (Furious Blows): Your movement speed increases after you land a critical hit.
Focus ability:
They Never See the Little Guy (Cloak of Shadows): You go into stealth, and your allies follow your lead. The entire party remains undetected for the duration of the effect, even while attacking.
Cole (Compassion)
Active abilities:
Fast as the Fade (Flashing Steel): You lash out around you with both daggers, hitting all enemies in melee range.
Catharsis (To the Death): You taunt an enemy into a frenzy. Their damage output increases over time, but so does the damage they take. The effect ends if you get out of range.
Peace (Blinding Shot): You hit an enemy with an explosive blinding powder, causing their next attack to deal no damage.
Passive abilities:
It Doesn't Matter that they Remember Me (I Was Never Here): You have learned to use the distraction of an enemy's death to slip silently into the shadows. Killing a target removes any cooldown on stealth.
Forget (Knife in the Shadows): You are a deadly threat to any enemy that can't see you coming. When you attack from stealth, you automatically critical hit.
Mercy (Throatcutter): A wounded enemy is the perfect target. Your attacks are even deadlier against targets that are close to death already.
Fade Mind (Unorthodox): A spirit's mind travels curious, twisting paths, and though his methods are strange, they are effective.
Focus ability:
Final Rest (Thousand Cuts): You choose a target, then dash to and fro- a shadow leaving blood with every leap. After you slice through other nearby foes, you land behind your target, striking deep.
Dorian (Tevinter Exile)
Active abilities:
The Stories are True (Horror): You unleash spirits of fear that terrify all enemies within the area.
I'll Remember That (Spirit Mark): You mark a target with an attacking spirit, inflicting ongoing damage. If the target dies while marked, the spirit mimics the victims body briefly to fight on your behalf.
Used to Criticism (Wall of Hissra): You create a wall of elemental magic before you, hindering enemies that pass through it.
Passive abilities:
There's Another Way (Defiant): The exile's dedication to the cause of reform spurs him to fight ever harder.
Tevinter sabotage (Control of the Elements): All status effects that you inflict last longer.
Waste Not Want Not (Death siphon): Every time an enemy dies nearby, you regain both health and mana.
Tevinter has a Bad Reputation (Blinding Terror): You have learned to leave enemies vulnerable in their terror. Enemies that are panicked take increased damage from all attacks.
Focus ability:
Progress and Reform (Haste): You increase the speed of the entire party. While this ability is active, all enemies move and attack more slowly by comparison.
Solas (Fade Walker)
Active abilities:
Lure of the Fade (Pull of the Abyss): You create a tiny rift that pulls enemies toward a central point.
Not All Spirits are Peaceful (Veilstrike): You recreate your own fist from the essence of the Fade and smash nearby foes to the ground.
Time Flows Differently There (Disruption Field): You fill an area with magical energy that slows and weakens your enemies. Enemies larger than the field are immune.
Passive abilities:
Self Reflection (Fade Shield): You draw back the energy released by your enemies in your attacks against them. Any successful attack strengthens your barrier. The more damage you do, the more powerful your barrier grows.
Calming Meditation (Composed): Long years of meditation have given the fade walker an unshakeable calm, even in the face of certain death.
A Different Perspective (Mind Over Matter): You are immune to flanking from enemies and have less chance of being staggered when hit from the front.
Calm of the Veil (Restorative Veil): You pull stray magic from around weakened enemies to regain mana based on the damage you do to them.
Focus ability:
Rage of Spirits (Firestorm): You summon flaming meteors, raining fire down upon enemies all over the area for the next several seconds.
Vivienne (Madame de Fer)
Active abilities:
A Girl's Best Friend (Spirit Blade): You create a blade of solid magic to make melee attacks against nearby enemies, bypassing their guard and barriers.
Fight with Style (Blessed Blades): You rally all of your nearby allies to fight with greater strength.
Blinding Fashion (Wrath of Heaven): You summon a blinding pillar of light that stuns and damages nearby enemies.
Passive abilities:
Bon Mot (Veiled Riposte): Whenever you have a barrier active, enemies who attack you will take damage in return.
Wicked Eyes and Wicked Hearts (Calculating): The court mage seeks ever more knowledge and power, and she is unafraid to use both in battle.
Keep Your Enemies Closer (Combat Clarity): The chaos of combat frightens some, but for you, it reminds you of the court. Your mana regenerates faster when you're near hostile enemies.
Don't Touch the Outfit (Psychic Backlash): Enemies that hit you with a melee attack have a chance to be stunned for a short time.
Focus ability:
Don't Tell me You're Already Tired, Darling? (Rally): You rally your allies to fight harder. Your party's guard, stamina, and mana build up over time, and all party members gain damage resistance against incoming attacks.