>>53763491Solid Rock or Overcoat (aka Dust Proof in moon runes, he IS mold, he feeds on dust) as an ability would make a lot of sense thematically, and gameplay-wise would solidify his role as an incredibly reliable suicide lead.
Mold Breaker would also be better than his current options, and it would be a funny ability for a mold Pokemon to have.
Contrary is cute, but does very little in the lead role outside of encoring a defogger to become a pain.
Sturdy is very nice for a lead, but Shuckle still dies get ohko'd by double iron bash, choice banded surging strikes, and choiced banded rock blast w/ 5 hits.
Gluttony is incredibly niche, you can run it in a meta where you're more worried about being ohko'd by lead Melmetal than you are single hit moves.
Solid Rock would make Shuckle virtually impossible to ohko on the first turn of a battle, it does everything sturdy does and better, save for repelling some lunatic running fissure/sheer cold/guillotine in AG.
Overcoat would make Shuckle immune to sleep powder and spore, which would be a boon in many different metas where Breloom and Amoonguss are ubiquitous.
Mold Breaker would be the ultimate fuck you to magic bounce users allowing Shuckle to set up sticky webs and stealth rocks, as well as encore through their bounce. It also helps with aroma veil, if you ever actually run into Alcremie or Aromatisse.
Some people want more moves or better stats, but Shuckle already suffers from a minor case of 4 move slot syndrome, and a defensive ability goes a long way.
Shuckle could probably move out of the lead role and gain usage as a wall if it had an extra 20 base hp and body press, but there are few sticky web users as good as Shuckle and I'd really like for him to get more tools for his role as a lead.