Heres what i can think of
1. Garchomp - SP ATK scales with ATK (cause some of its coverage moves are special atk such as fire blast)
2. Golisopod - Speed is raised to 2x or emergency exit activates IF the next attack to golisopod will send its HP below 50% or will knock it out also the switched in pokemon will take no damage from the incoming attack
3. Aggron - SP ATK scales with ATK (Cause it can learn many good special attacks such as thunderbolt, ice beam, flamethrower and surf)
4. Medicham - SP ATK stat Huge Power ( I Just wanna see a special attacker medicham for once)
5. Vespiqueen - All order moves always goes first even before protect
6. Charizard - Immunity to rock moves and stealth rock
7. Venusaur - All moves that uses leaves gets 20% boost also increases spd stat by 10%
8. Blastoise - Mega Launcher for all Special water moves
9. Sceptile - Overgrow gives grass moves 100% boost instead of 50% and takes 20% less damage
10. Blaziken - All kicking moves are boosted by 15%
11. Swampert - When hp is below 33% Ground moves have 50% boost (Torrent for ground moves you could say)
12. Slaking - Takes 30% less dmg or Truant is negated
13. Wobbuffet - Forces enemies to use only attacking moves if none they use struggle
14. Tyranitar - Takes 50% less dmg from fighting moves (Basically reducing his weakness from it to 2x only)
15. Wishiwashi - If held wishiwashi is in alone form when sent in battle and goes to schooling form once it has taken dmg (even just one hp dmg) throughtout the entire battle (if spikes or stealth rock is active or weather dmg is taken this is immediately activated)
16. Gyarados - Atk & SP Atk is Equalized ( So i can finally use flamethrower and surf for this guy)
17. Pidgeot - Contact moves that has 60 power or below procs twice ( Aerial ace, Quick attack, etc strikes twice)
18. Dragonite - Gives the other ability (giving it both Inner focus and multiscale)
19.. Snorlax - Gives Comatose
20. Aegislash - Automatically switches to shield form if it will be attacked
21. Malamar - If in battle all stat changes of other pokemon are reversed
22. Zoroark - Illusion wores off after 3 attacks ( Could be useful if the AI takes zoroark as the pokemon he transforms into)
23. Kommo-o - Sound based attack moves are boosted by 30% while sound based status moves have their effect doubled ( Such as screech and noble roar)
24. Goodra - Gains water absorb and causes all water attack moves to slow enemies by 1 stage (Surf, muddy water, etc)
25. Steelix - ATK and DEF is equalized (Gotta use that extra def stat somehow)
26. Lucario - If it takes no damage in a turn then it gains the effect of focus power on the next attacking move it uses
27. Shedinja - Upon switching in the 1st move it uses is always a critical
28. Lopunny - Gives Normalize and causes all of its normal type attacks to do normal damage to all types except ghost ( Meaning all its attack will be normal type that deals normal damage and disregard enemy typing but ghost)
29. Flygon - Causes flygon to be a Dragon/Bug type instead of Dragon/Ground
30. Walrein - Gives Aqua ring upon switch-in and Sheer cold has 2x accuracy in Hail
31. Haxorus - Upon switch-in both Defenses are raised by 1 stage
32. Araquanid - The first physical attack against this pokemon is negated (Because its protected by its bubble)
33. Hippowdon - Upon switch in a sandstorm brews and lasts as long as hippowdon is in the fight (So he can use his sand force ability. I want the sandstorm gone if he goes down because if its still there then other pokemon with sand force can just sweep the enemy)
34. Rhyperior - Rock Wrecker does not need to be recharged (If you think this can make him broken well i think not because a 4x dmg can just knock him out right away or not because of solid rock ability but its still stoppable)
35. Dragapult - Disables the first damaging moves used against it
36. Clawitzer - Improves mega launcher from 50% to 100% boost
37. Dragalge - Increases chance of critical hits and/or Turn it into Poision/Water type
38. Aurorus - Raises evasion by 2 stages in Hail
39. Tyrantrum - Contact moves are boosted by 20% (Making head smash kill almost anyone in a hit )
40. Salazzle - Poison moves have chance to burn and Fire moves have chance to poison enemies
41. Abomasnow - Fire moves are negated in Hail
42. Beedril - Defense is TRIPLED
43. Gardevoir - Attacking pokemon that makes contact has 50% chance to be infatuated regardless of gender and even if they are genderless
44. Incineroar - Taunts enemies into attacking him as long as he is in the field
45. Archeops - Negates its ability
46. Magnezone - Activates magnet rise upon switching in
47. Reuniclus - Trick room activates upon switching in
48. Camerupt - Eruption is always at max power
49. Houndoom - Burn chances are double and Burn from houndoom cant be removed (Dex entry inspired)
50. Salamence - Outrage ignores the chance of hitting itself in confusion (Dex inspired)
51. Metagross - If Metagross moves after its target its attack has 25% boost
52. Shiftry - If enemy moves before shiftry, Shiftry has 50% chance to avoid the incoming attack. Lowers base speed by 20 but increasing both base defenses by 10
53. Escavalier - if there is an accelgor in battle Escavalier's speed will match Accelgor's Speed (might be good in double battle)
54. Accelgor - If an escavalier is in battle accelgor will match escavalier's Defense (might be good in double battle)
55. Arbok - All biting moves poison the foe
56 Tentacruel - Defense scales with Special defense and attack scales with special attack
57. Golem (Both forms) - Once knocked out it causes a self-destruct
58. Poliwrath - Increases dmg of contact moves when raining by 25%
59. Kingler - HP is doubled upon entry
60. Shedinja - Requires 2 supereffective hits before going down and makes it immune to status and weather effects
61. Wailmer - Water spout always at max power
62. Bewear - Gains scrappy for contact moves and ghost type get 50% bonus damage from its contact moves
63. Drifblim - Burns drifblim and doubles its speed stat
64. Golurk - Doubles golurk's speed
65. Chandelure - Flash fire is activated
66. Minior - Swaps the effect of its ability ( Shields are down upon more than 50% hp then shield are up when hitting 50% or below hp also removing status effects)
67. Gigalith - Once knocked out it leaves a stealth rock or spikes at the enemy side
68. Scizor - Allows it to negate the 1st fire move used against it and doubles its speed after doing so (Works only once per battle)
69. Heatran - Adds the effect of field changing fire moves to the field (if applicable) upon entry
70. Bisharp - Allows it to negate the first fighting type move used against it and increases its attack stat by 1 stage
71. Octillery - Upon taking a contact move it lowers the attacking pokemon's accuracy by 1 stage & increases defense by 1 stage
72. Kingdra - Summons rain as long it is in battle (Like hippowdons')
73. Ludicolo - Gains Dancer
74. Sharpedo - Always moves 1st in watery fields also gains strong jaws
75. Galvantula - Uses sticky web upon entry and other web moves gain priority