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Mega Heatmor (484 BST --> 584 BST)
Ability: No Guard or Infiltrator or Simple
HP: 85
Attack: 97 --> 107 (+10)
Defense: 66 --> 76 (+10)
Sp.Attack: 105 --> 140 (+35)
Sp.Defense: 66 --> 76 (+10)
Speed: 65 --> 100 (+35)
Not every Mega Evolution is supposed to suit a meta mostly polluted by the same overpowered Pokemon. You are free to speculate on single stage Pokemon possible megas and edit their sets, but please, try to be realistic with stats and abilities or at least explain them.
I wanted to keep Heatmor's mixed offensive possibilities and allow it to either be a one trick pony with Inferno and Focus Blast spam or a mixed Work Up abuser with Simple. Infiltrator is listed in the possible abilities because of its specie that is the Anteater Pokemon, I suppose this Pokemon knows how to abuse its tongue and wreck through Substitute.
Flash Fire could remain as a possible option too.
It's an inferior Victini or Delphox but would have its own niche because of the moveset, base 100 Speed may look too much but it is considered mediocre speed in Gen6 so it was already a gift for it. The abilities may look way too good even if those could even fit it however its bulk and defensive type makes up for the newly acquired power.