First off, give him Power Gem and maybe some strong special signature move that's rock type, since the best special STAB he has is Ancient Power. Give him Sturdy(Replace Magma Armor)+Metal Burst/Mirror Coat and he's now basically Bastiodon, another mon harassed by common water, ground, and fighting moves, except Magcargo has a usable offensive stat. Also now he can play weakness policy or metal burst mindgames. Sturdy also enables it to take a hit and use Z-Memento, allowing a sweeper to come in safe and set up if Magcargo is at full health against a revenge killer. Baton Pass also lets Magcargo slowly switch into something else safely, as well as pass on a boost from one of its many set up moves. Give him a Special variation of Explosion like how they introduced Giga Impact when Hyper Beam became Special, maybe fire type instead of normal since, you know, explosions. Also, Eruption makes sense given the fire spewing upward from the pore in his shell. Can also function as diet Golem via Sturdy with stuff like Custap Berry Explosion/Special Explosion, Stealth Rock, or Attack with a strong move like Overheat. It's completely outclassed still, but Darmanitan's not a bad pokemon just because Victini exists. Sturdy Pain Split can also take a chunk out of an opponent, depending on how much HP they have. Give 20 extra special attack and primarily use Magcargo against walls, either setting up in their face with its various stat boosting moves or using clear smog to stop stockpilers and the like in their tracks. Another potential ability would be innards out or aftermath, since I imagine defeating a combusting snail could result in an explosion of rocky debris.
This one's for you, Ivan Ooze and Melvin Tooz. Happy Birthday.
