This seems really fun, so I'm gonna try this out.
I'd like to submit
Shell Smash Slugma with +5 base Speed (20 -> 25).
Slugma @-Eviolite
Ability: Flame Body
Level: 5
EVs: 36 HP / 36 Def / 196 SpA / 236 Spe
Timid Nature
- Shell Smash
- Flamethrower / Lava Plume / Fire Blast
- Earth Power / Ancient Power
- Clear Smog / Recover / Will-O-Wisp
It's clearly a shame that Magcargo has access to Shell Smash and not Slugma, because Magcargo has really no place in any meta with it thanks to its terrible speed. However, Slugma could make waves in LC easily with access to Shell Smash, and I think that simply getting that move warrants an LC Theorymon post.
Flavor wise, it works out nicely. While Slugma's cooled, rocky exterior upon slowing down isn't exactly a literal "shell", Slugma would honestly need some kind of way to, perhaps,
smash, or otherwise shed, that shell to prevent it from dying a slow and cold death.
After Shell Smash, it hits a respectable 26 speed, which outspeeds the entire unboosted metagame and most Scarfers. It also has a variety of qualities it can abuse to distinguish it from other Shell Smashers. Slugma's unique Fire/Ground coverage allows it to roast the competition, plus with Ancient Power and Clear Smog, it has multiple options to take on different types. It also has utility in Recover, Will-O-Wisp, and Clear Smog to disrupt opponents and stay in the game longer. Flame Body as an ability also means it can discourage priority to an extent, and its typing is excellent for a Shell Smasher because it isn't weak to Fighting like all the others. Its decent bulk with Eviolite allows it to set up fairly easily, though it's going to have a hard time against Diglett, Tirtouga, and Archen. After a boost it can OHKO bulky Chinchou, bulky variants of Mienfoo, and even has a shot at OHKOing Timburr with Fire Blast. And don't forget, Slugma isn't setup bait either, thanks to Clear Smog.
I'm not sure if Shell Smash would make Slugma broken or not, but it would certainly cause its viability to skyrocket, or, at the very least, give it something to do aside from "sit around and use Will-O-Wisp".
EDIT: I forgot to mention that it has really good synergy with Vulpix, as Sun makes its fire STABs much more potent. OHKOing Timburr and Pancham with Fire Blast is easily possible under sun.