Affixes: Make your own!

[OrcinusDrake]

Brostorm - Friendly NPC follows the closest party member doing AoE damage to enemies. Enemies can't be reduced below 1 hp, but will then die when hit by NPC.
Goo is love, goo is life - Void zones coat the instance. Taking a few ticks of damage from them clears that area, so cut a path through.
BFFs - NPC sticks to you (like hexos), dealing direct damage to it or healing it (for heal specs) increases your stats up to 120% of your normal str/agi/int. Ignoring it causes those stats to slowly tick down to 0.
Cauterised - Reduce your healing taken based on overheals taken in the past 60 seconds.
Forrest gump - One of your party members now constantly runs forward. Can be transferred via extra action button.
Skinny dip - You are not allowed to use a certain gear slot during this run.

[Cerilis]

I was inspired to another idea.

Vanilla Icescream - Every class gets a major setback, that was a feature* in vanilla.

* Like, rogues have poison stacks and any combo points reset when you dare to click on another target (friendly included), mages have to drink after every group, shamans got to use totems for their buffs and can't relocate them, paladin buffs only last 5 minutes, dks, monks and demon hunters won't be able to play, etc.

[Enter Name Here]

Short-Sighted - Enemies and players break LoS
Drunk - Inverted movement
??? - harm-/helpful spells scale with the life-percentage of the caster
do your effing job - getting hit by avoidable stuff grants an x% debuff on dmg/heal which lasts for the entire run, stackable

[Trickname]

It's not safe back there: Take ticking damage if more than 7 yards from the mob / boss


Or anything to balance what will most likely be the case of melee being a hindrance.

[Deleted]

Control is king! - Non-boss enemies apply a debuff, where a hit from another unique non-boss enemy kills the target. Crowd Control (other tactics) must be used to reduce the enemies to one at a time.

Foggy - The dungeon is cloaked in fog, hiding enemies in plain sight. Tread carefully and use your memory, you might pull more than you can handle. Cast range reduced to 20 yards.

They Rise! - Upon each enemies death, a ravaging ghoul or exploding shambler spawns. The ghoul does increased damage for each death that occurs whilst it's alive, and shamblers slowly walk toward a random target, exploding upon death for X damage.

Die as one - Enemies must be killed within 3 seconds of each other, else they will return to life with full health.

[Rehok]

Leeeeerooooooooy Jeeeeenkins - A DPS now dictates when stuff is pulled as all mobs are now immune until they damage them and mobs just randomly pull from other packs, if you complete it within the time limit you get some chicken

or something like that

[Arkura]

"Uncontrolled" Mobs and bosses will reset threat every so often.
"Spikes" Bosses will passively deal 5% of maximum HP every 4 seconds.
"Mana Drain" Mobs will drain some mana from random party members.
"Extra protection" Bosses have mobs with them that has to be killed before boss can be attacked
"Focus" Mobs will focus one player for X amount of seconds. Not tauntable.
"Knockback" Mobs have the ability to knock a player back, stunning them for X amount of seconds dealing X amount of damage.
"Fire chase" Normal mobs will spawn fire underneath a player every second while player is standing still for 30 seconds. Debuff lasts 8 seconds. Player recieves 50% increased movement speed for the duration.
"Time to go boom" Mobs and bosses gain the seed of corruption ability, when an affected player has taken X amount of damage the debuff will explode dealing X amount of damage in a 12 yard radius and leaving a corrupted area on the ground, players standing in the area will take damage.

If I continue thinking about this I could probably think about some better ones :P
I was trying my best to stay "melee friendly".

[Zanejk]

Keeps Your Enemies Close... - Non-boss mobs have an increased aggro radius
...And Your Enemies Closer - Non-boss mobs deal dynamic ticking damage to the entire group, with more mobs in the instance currently aggro'd decreasing the damage
I'm So Confused - During combat one player every 20 seconds will be Mind Controlled, breaking after 5 seconds or they receive damage equal to 10% of their health
Consumption - A main stat, rotating every 5 minutes, is lowered for players by 25% while being increased for all hostile mobs by 25%
I Drink Your Milkshake! - All healing players receive is spread evenly amongst all engaged mobs, with all heals bringing a player over 80% health being tripled and applied to all hostile mobs
Me So Thorny - Occasionally a player will receive a debuff, wherein they will take 35% of the damage taken by all engaged mobs for the duration of the encounter
Center of Attention - Players will randomly receive, during combat, a buff that dramatically increases their threat generation
Calloused - Every ability cast against boss mobs will apply a stacking buff to the boss, giving them a stacking damage reduction shield that drops a stack every 2 seconds the boss does not take damage, stacking up to 25%

[Nadiru]

Extra Credit Bonus Level: The last boss in the instance is locked behind a door which requires five Golden Orbs (hidden throughout the dungeon) to open.
Big Red Button: A big red button spawns with each boss. At 30%, the boss will become invulnerable and press the button, causing him to use spells and abilities much more frequently and for more damage.
Gauntlet Mode: Healing spells have 30% reduced effectiveness; Dungeons spawn health pickups after each room.

[schwarzkopf]

SMART : Non boss enemies have a 20% chance of focusing on the healer until it (or the healer) dies.
LOOKERS : Non boss enemies aggro on line of sight, not range.
LISTENERS : Non boss enemies can aggro if nearby groups of mobs are being attacked.
HUFF : If at any time 3 consecutive interruptible boss abilities are interrupted, the boss vanishes in a huff - returning/resetting the fight 30 seconds later.

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but... everyone can do this content?

Just like challenge modes in WoD - something I don't have access to. Doesn't worry me, lots of other stuff to do - but without a bunch of regular players, this stuff is out of bounds for many of us.

[Deleted]

Rose-tinted glasses - Trash packs respawn after <time>. Something like 5 to 10min.

[Sierra85]

Can't Stop: all players are constantly moving.

Memories: after defeating a boss, another random boss from a previous expansion spawns at full health and max level. Players must defeat this memory.

Night Time: Players can only see 5 yards infront of them and trash now patrols patrols differently.

Wishes: A genie grants each player 1 wish after killing each boss, but they can backfire.

[Flurryfang]

Legion Themed Affixes!!

Invasion!: During combat, random portals open up to the nether, spawning random demons to aid your enemies. When killed, the demons leave behind a benefitial portal, which upon activation, will buff a player for a moderate amount of time.

Doom!!!: All players get afflicted with DOOM! at the start of the instance. DOOM! deals a large amount of dmg every 30 sec.

You don't have to be so Ill: All bosses now spawns 2 flame imps for each 1% of HP lost. These imps have low health and shoots fireballs at 3 random players a small amount of damage.

All Hail The Prince: During boss fights, Searing Infernals rain from the sky. These Infernals cast hellfire, dealing large amount of dmg in a 15 yard radius. These infernals can be killed.

[Unholyground]

Arcane Torrents: Add a 5% chance on attack to shoot 3 arcane bolts at random to party members for 15% of their current HP.

[Kajitheninja]

Living Dead:When a mob or add die, they return to 25% of their max HP
Empowerment:When a mob dies all mobs in a 20 yard radius get a 25% damage buff (stacks)
The Wall:Specific mobs throughout the dungeon pick a random target to surround in a wall causing line of sight for X seconds
Critter Army:Mobs cast an instant polymorph at a target that lasts X seconds
Melee Life:Party members have to stand in melee range at all times in order to do full damage. (anything farther than melee gives players 50% reduced damage)

[Calamari]

Arcane Torrents: Add a 5% chance on attack to shoot 3 arcane bolts at random to party members for 15% of their current HP.

Level 8+ Affixes:

Phase Horrors - Invisible adds patrol the dungeon and facehug the first player they find, stunning and damaging them until killed. Fairly low health but move fast and respawn every ~3min

Temporal Anomaly - Your melee haste and casting speed fluctuate between -30% and +30%, shifting like Incanter's Flow.

March of the Murlocs - D.E.H.T.A. is saving the murlocs and has rerouted a massive murloc migration through your dungeon, every ~3min a stampede of murlocs comes storming through the entrance portal and through the whole dungeon, contact with the murlocs deals low damage but massive knockback to all players and in-combat mobs (except bosses).

Kael'thas' Revenge - Kaelthas is back and joins in for the final boss fight to turn your world upside down, now you're swimming through the air while being chased by murder orbs and Kael is spamming pyroblasts at the healer, enjoy.

[avahle]

1. A debuff that damages you for 5% health every second you are standing still anywhere in the dungeon. Might be brutal for casters, but will still be fun.
2. You need to carry a "Eye of Anzu" type item from start to finish. It applies a stacking damage debuff, so you will need to keep passing it around.
3. Applies debuff at random times to random players throughout the instance. If dps gets debuff, damge they do goes to tank. If tank gets debuff, threat they generate goes to healer. If healer gets debuff, healing they do goes to targets.

[Manhands]

Charming: If more than 15 seconds passes without dealing damage, you are mind controlled for 6 seconds.

Have to train packs to each other, healer has to do damage.

[Turtlewithnoshell]

"Deja vu" doesn't really add challenge, just busywork.

"I just love that run back after a wipe" (or a victory according to your suggestion) is something nobody said, ever.

It's pretty hard to come up with even vaguely interesting things to add to mobs that hasn't been thought of already (enrages at low HP, bigger aggro radius, stuff like that), i can't think of anything not too obvious, TBH

How about, Deja vu making each boss have an extra ability from the previous boss?

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Tag Team Bosses randomly swap during fights

[UnluckyAmateur]

I was thinking about one where you can aggro mobs if you're in their Line of sight.