If I only had one buff/nerf for each character:
Less endlag on Whirling Fortress in general.
Fsmash buffs. Especially the hitbox, because it wiffs too much.
Less landing lag on Fair/The Knee.
Less landing lag on Fair/The Claw.
Electroshock Arm sends opponents at a lower, more horizontal angle.
Gordo buffs. Either pre-patch Gordos making them harder to reflect, less endlag on the Gordo Toss itself, or etc.
Uair nerfs. Comes out slower (frame 3 is just silly), less damage/knockback, etc.
Kong Cyclone nerfs. In particular removing/nerfing the super armor it has.
The ability to snap to ledges almost immediately after the last hitbox of Down B comes out.
Improved linking on the hits of his Smash Attacks. Especially Fsmash.
Less endlag on Blasters.
Less endlag on Shine/Reflector.
Jab buffs. Less startup and endlag in particular. Maybe an improved hitbox too. And increase his grab range as well.
Faster Usmash.
Less startup lag on Fair.
Less endlag on his Smashes.
Less endlag on Dair.
Very slight improvement in aerial speed.
Improved shield stun/damage on his Smash Attacks. Especially Fsmash and its variants.
Improved knockback on the final hit of his aerial Up B.
A little bit less endlag after landing on the ground during his vulnerable aerial state.
Damage/Knockback improvements throughout her moveset, to make her moves safer on shield and her benefits more pronounced in comparison to Marth.
Changing the tapping B button function of his Luigi Cyclone to holding B instead, so people don't break their controllers.
A small increase to Uair's damage.
Slightly increased running speed on the ground. Also applies to Lucina. (Because I'm a cheater like that
)
Make the sourspot hit on Flame Blast last all the way to the end of the animation, to make the move safer.
Less startup lag on Fsmash.
Dair buffs. Especially to its startup lag.
Removal of the Order Tackle glitch.
Removal of the infinite Lightweight glitch.
Grab hitbox/duration buffs. Having the grab pull people in during the entire animation, and not wiffing at the furthest range.
Adding a sourspot on her Bair that will last until the end of the animation, after the sweetspot.
Heavy Skull Bash nerfs. In particular, removing the ability to quick-charge the move by smashing it.
Less endlag on his grounded Arrows of Light. Also applies to Dark Pit. (Because I'm a cheater like that
)
A bit less startup lag on Nair.
Less landing lag on his Nair.
Shooting Star Bit nerf. Far damage reduced from 5% to 4%, medium-close damage recuded from 4% to 3%.
Dsmash buff. Have it send opponents at a higher angle with increased knockback. (Easier edgeguarding for Charge Shots)
Increased landing lag on Fair.
...I honestly have no idea?
Damage nerfs across the board. Especially Spin Dash/Spin Charge -> Nair. +30% damage is just nonsense.
Improved knockback on the final hit of his aerial Up B.
Extreme Balloon Trip nerf. Grabbing the ledge results in the balloons disappearing instead of exploding.
The ability to jump off of his bike while preforming a wheelie.
Improved hitboxes in general, to better help her hit small characters.
Increased damage on his throws in general.
Less endlag on her aerials in general.
If possible, decreasing the hitstun on Uair without nerfing anything else. Either way, a small nerf to Uair.