Rebuild it! Abomasnow and Mega Abomasnow

Replace Soundproof as an ability (only super-effective moves are Chatter, Bug Buzz and Metal Sound) with either Thick Fat (a more reasonable option to cut down on its Fire-type weakness) or Solid Rock/Filter (this would make it OP.)

 

I used an algorithm to find suitable abilities. Thick Fat stood out the most.

Spoiler: Here are the other (that I consider) thematic Abilities available, for anybody curious (very long) :

• Anger Point
• Berserk
• Damp (soft)
• Defiant
• Fluffy (oh hell no)
• Friend Guard
• Frisk
• Fur Coat
• Grass Pelt
• Guts
• Harvest
• Huge Power
• Hustle
• Ice Body
• Inner Focus
• Intimidate
• Mold Breaker
• Moxie
• Oblivious
• Overcoat
• Overgrow
• Pressure
• Pure Power
• Reckless
• Refrigerate (OP)
• Sheer Force
• Snow Cloak
• Snow Warning.
• Sturdy
• Unnerve

Told you so.

 

I have no idea about the competitive battling scene, but I can say that its biggest weights are its typing (among the worst in the game) and lack of speed. It's got weaknesses to Fire, Bug, Poison, Steel, Rock, Flying, Fighting and its sluggishness means that a lot of Pokémon are going to get the drop on it and it checks few Pokémon.

Here are some solutions:

• Buff Speed, reduce SpecAttack: I get that a tree is supposed to be slow, but even a 20 point increase would be an immense help.
• Remove Soundproof: Soundproof does little for it. Most sound moves aren't much power beyond something like Hyper Voice or Clanging Scales. I can name several other abilities that would be more useful in its place. Heatproof is a solid pick since some evergreen tree species are actually fireproof like redwoods (fires take place INSIDE the trees) This can also help reduce the fire attackers, but they still remain a problem. Refrigerate would make this thing crazy OP and with the speed buff above, it can decimate a lot more.
• More priority moves: I like Ice Shard, but adding some others like Sucker Punch can catch people off guard.
• Physical ice moves: Give this thing a stronger physical ice move like Ice Hammer or Icicle Crash.

 

I think Snow Warning is a fine ability for Abomasnow and its Mega. Perhaps as a hidden ability instead of Soundproof it could have Refrigerate or Adaptability.

Stat wise, I think it would be helpful for Abomasnow to have increased Defense, Special Attack, and Special Defense. Giga Drain and Blizzard are two of its best stab moves imo and those buffs would help improve it while not making it drastically OP. Since it has a BST of 494 (mega 594) I'd change it closer to Gyarados' with 540 and 640 for its mega. With an additional 46 stat points, I'd add +15 to Def, +15 to Sp.Def, and the remaining +16 to Sp.Atk.

Meaning that Abomasnow's new stats would be
Hp 90
Atk 92
Def 90
Sp.Atk 108
Sp.Def 100
Speed 60

(Mega)
Hp 90
Atk 132
Def 120
Sp.Atk 148
Sp.Def 120
Speed 30

Which seems fair considering the multiple amount of weaknesses it has.

I think something that would also help is if the Hail weather condition was buffed as well. Similar to sandstorm, I think it would be fair for Hail to boost Ice Type's special defense stat by 1.5x. This would mean that Abomasnow's Sp.Def would be 150 (180 for mega) with hail activated, making it rely more on weather as its niche.