This would be so cool right? You could actually get a switch in on a fire attack, raise your power with flash fire and mega evolve which would be REALLY COOL. So after Mega evolving mega arcanine remains a pure fire type and gains the ability white smoke, which eliminates stat reduction, allowing you set up flame charges and agility without getting your stats lowered and also nullifies intimidate and bulldoze effects. It gains boosts to its sp def, sp attack, speed and attack. Its new appearance is a more powerful, more regal version of its old self with fiery, whitish smoke enveloping its surroundings.
Stat boosts:
Sp defense 40
Sp attack 20
Attack 20
Speed 20