If you could make your own ability...

[Gh0stStark 30]

Adamant Armor - Reduces the damage received from Fighting or Fire type moves by 75%.

Magma Body - Grants total immunity to Rock type moves. Can't be negated by any other ability or move.

[Jmanultrax1 38]

luminescent: when fighting a dark type opponent allows the user resist dark type moves.

[PrimeMehster 0]

Martial Arts: If user uses contact move of same type as opponents move (has to be an attacking move, not status), reduces damage of opponent's move by 66%, and reduces your move's damage by 25%.

(Not sure if this would be imbalanced or not, but basically this ability would reward the user with increased longevity by correctly anticipating the opponent's playstyle, but could also reduce damage output at the same time.)

Warden: If the opponent is "trapped" (wrap, infestation, sand tomb, etc), user gains instant priority on every move until opponent switches out of trap effect. (Does not count on ingrain or moves like leech seed).

[IntSys 106]

Checkmate: The user's moves' power gets doubled if the opponent's switching while having equal or less than 50% HP.

Staleness: The user's moves gradually decrease in power by a tenth every time it's used in a row (Capped at 0.5x). Resets if the user uses two different moves in a row. Resets if the user switches out.

Wrecker: The user is guaranteed to land a Critical Hit against a Pokemon with Defense or Special Defense boosts.

[Sapphire Mahala 62]

Vanish Plot

user temporarily switches out with special type attacking pokemon and it takes the damage instead if physical switches with enemy pokemon party member

[Alilatias 53]

Momentum: The Pokemon's speed increases by 1 stage after every use of multiple turn attacks with increasing power. Speed boosts are lost as soon as the attack ends. (Such attacks include Rollout and Ice Ball. More such moves should be created for other types. Speed loss can be negated by White Herb.)

Rolling Shield: Non-contact moves inflict 50% less damage if the Pokemon is currently using multiple turn attacks with increasing power.
Tight Grip: Damage inflicted from binding moves is doubled each turn.

[Ice Cream Sand Witch 1650]

My recent pun status got me to thinking about this.

An ability that makes the Pokemon immune to attacks with beam in the name. Maybe it could be called Reflection?

Beam moves are Psybeam, Charge Beam, Signal Beam, Simple Beam, Bubble Beam, Aurora Beam, Ice Beam, Hyper Beam, Solar Beam, and Moongeist Beam.

[Govictory 0]

because you can never have enough edge

Edgeism - dark and fire type moves do 33% more damage to non dark or fire type pokemon

[Lugruf 50]

Unmega: When this pokemon enters, transform all mega evolutions to their base form. Pokemon can't mega evolve.

[IntSys 106]

Suppression: Deducts the BP of any move the user uses and adds it onto the move's Accuracy until it reaches 100. For example, Blaze Kick becomes a 75 BP 100 Accuracy move while Stone Edge becomes a 80 BP 100 Accuracy move. Only works with attacking moves that have BPs and Accuracy.

Brute Force: Deducts the Accuracy of any move the user uses and adds it onto the move's BP until it reaches 100. If the move already has more than 100 BP, then the Accuracy won't be deducted. For example, Bullet Punch becomes a 100 BP 40 Accuracy move while X-Scissor becomes a 100 BP 80 Accuracy move. Only works with attacking moves that have BPs and Accuracy.

[Bessi 2]

Aura Shield-decrease the damage of all special moves by 50%

Super Charger-all 2 turn moves are executed in one turn but they take two PP

[Shinyrio 8]

I got this one last night while thinking about bringing my Pokemon's potential to the max.

Power Awakening - A Pokemon with this Ability gains an additionnal STAB on all moves of a predetermined Type. This type is determined by the same method which determines the type of the move Hidden Power. If the Pokemon already has a STAB on this type, the Pokemon is treated has having Adaptibility for this type.

The question is : how could I make this Ability works for Fairy and Normal types...

[Jmanultrax1 38]

elemental shield: moves that do double damage against the user do 1.0 damage instead of 2.0.

[Kreign 0]

The ability to remove Wimp Out because seriously, that ability sounds awful.

Joking aside,

DriveTheft: Steals the opposing Pokemon's stat changes caused by a damage inflicting move.

Applies to when an opposing Pokemon, for example, uses Power-up Punch against a Pokemon with this ability. The Pokemon with the ability would steal the attack buff. also applies to Contrary and Debuffs such as Draco Meteor. Buffing you or debuffing you as it would. It wouldn't apply to set-up moves such as Calm Mind or Swords Dance

[Jmanultrax1 38]

driftor: every turn there is 40% chance that the user can avoid an attack entirely

[Lugruf 50]

Death aura: While in battle, prevents all healing for all pokemon

Guardian: The first time this pokemon would faint in a battle, it remains at 1 HP and switches out instead.

[Character OS 0]

Not sure if any of this has been done before, but...

Mystic Power: Doubles the Special Attack of the Pokemon with this ability.

Psychic Connection: Doubles Speed under Psychic Terrain

Limit Break: Increases Attack, Defense, Special Attack, Special Defense, Speed, Evasion, and Accuracy by 1 stage when at 1/3 HP or less. Decreases stats by 1 stage when healed above 1/3 HP.

Paladin Soul: If an allied Pokemon is targeted by a super-effective attack, the Pokemon with this ability will take the attack in its place.

Demon's Rave: All Dark-type attacks have a 20% chance of flinching.

Melt Swarm: Fire-type attacks ignore barrier moves and opponent's stat changes, breaks Light Screen and Reflect, and eliminates the opponents' Defense and Special Defense boosts.

Seraphic Whisper: Heals 12% of the Pokemon's HP when it uses a Fairy-type move.

Noblesse Laser: When this Pokemon connects with an Electric-type attack, 20% chance of repeating the attack.

Sprite Strain: Increases the power of all allied Pokemon attacks by 30%, except the Pokemon with this ability.

Light Barrier: Upon being sent out, creates a barrier on the user's side of the field nullifying damage up to 20% of this Pokemon's max HP. Fails if a Light Barrier exists already.

Doomsday: Increase the power of all damaging moves by 80%, but the user takes one-half of all damage dealt as recoil.

Crossing Cosmos: If an opponent attempts to use a move that this Pokemon knows, it will fail.

[CaptainMetal 17]

This would be the ability I would wish for a potential Mega-Deoxys:

Perfect DNA: The Pokemon switches its form according to the type of attack used.

Damaging attack -> Attack Form: Increased offensive stats, decreased defensive stats, ignores opponents stat changes

Passive attack -> Defense Form: Increased defensive stats, decreased offensive stats, immune to status conditions

Priority attack -> Speed Form: Increased speed, even defensive and offensive stats, increases critical hit ratio by two stages

The mechanic would be the same as Stance Change.

[Jmanultrax1 38]

Howling stance: the pokemon changes forms depending on the time of day. its stats also change as well

day form: pokemon has decent attack, defense and high speed.

night form: pokemon has higher attack, decent special defense, defense lowers and speed is lowered a little.

this is inspired by lycanroc.

[laggless01 51]

Determined - "Fights harder when under pressure." Power of attacks changes depending on HP: 100-75%: x0.5; 75-50%: x0.75; 50-red: x1; lower: x1.5 . Would be nice for some kind of Mega-Archeops to balance Defeatist and create strategy with it.

[Jmanultrax1 38]

weighed body: the heavier the pokemon is, their defense stat is increased. maximum of 3%

[Meio 0]

If I had to create one maybe it would be the non contact version of static and flame body, since only physical damng pokemon have to suffer from it.

It would be fun/infuriating if the electric/fire pokemon could have both of them, so everytime you had to fight a Magmar or a Raichu you would always have a 30% chance to get paralyzed/burnt (except with attacks like toxic or leech seed).

[Dypatome 58]

I'm not sure what to name it (maybe Deathly Touch?) but an ability that works like force palm would be nice. I'm thinking that every time a physical move is used, around 0-40% of damage done would be given to the user as HP.

[Jmanultrax1 38]

power strike: when the user's health reaches below 30% their next attack does an additional 5% damage. This can only activate once

[IntSys 106]

Spreader Surge: All non-status moves used by the user are now spread moves that hit both foes and allies.