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What Heatmor needs without giving it an evo would be a complete stat+ability rework. I'd make it a dedicated special attacker myself, as that'd fit with the flavor of it being a natural enemy of Durant; developing high special attack to bypass Durant's high defenses. I'm talking -20 Attack into Special Attack so it's now got a respectable 125 Special Attack. Next it needs new abilities; Flash Fire, White Smoke and Gluttony are all basically trash. Technician (would make Incinerate have comparable damage to Flamethrower while having good utility. It doesn't add much else now that Hidden Power is dead but it's something), Drought (obvious buff for a Fire type), or Solar Power all come to mind as potential abilities to make this work.
Or if you wanna make its physical side work because the thing has a way better physical pool, trade 30 Special Attack and put it into Attack then give it Tough Claws. The thing has gnarly claws, I could see it working, and obviously that works for fluffy moves like Fire Lash. And just for argument's sake let's post some calcs.
252 Atk Life Orb Tough Claws Heatmor Fire Lash vs. 240 HP / 52 Def Mew: 207-243 (51.6 - 60.5%) -- 91.8% chance to 2HKO after Leftovers recovery (Attack changed to 127)
252 SpA Life Orb Technician Heatmor Incinerate vs. 240 HP / 0 SpD Mew: 187-220 (46.6 - 54.8%) -- 9% chance to 2HKO after Leftovers recovery (Special Attack changed to 125)
Either of these options still leave it slow and frail but at least it'd be able to have a stronger foothold in low end play, even if it's not the top of OU or a VGC star, and without giving up too much fluff.
