Unlikely to happen.

Not only do flames have settings to do this, you can also control the alpha of your particle texture.

Adding a slider would not only cause GPU rape (you're talking doubling the workload for alpha blending for every particle on screen which can easily hit millions of both players have 5 maxed out flames), it'd also screw up the effect in general.
It may seem like a pretty basic thing to poke at, but layering multiple alpha settings on a single object is one of the quickest ways to drag even a top end GPU into the dirt.

Things like this is why the game has default settings to hide flames, trails, 3D objects and anything else distracting during matches and still show them on replays without having to do any opt commands.