When you counter these type of flames which either clog up the whole screen or cover the head to the point you can't see anything, the first thing you'd probably do is to /opt particles 0 [or turning it off in the settings], but what if you don't have to?

What if there was an option that makes ALL flames transparent with a way to set how transparent you want it to be, whether through a slider in the options or /opt particles 0-255[0 = off, 255 = opaque, all numbers between those indicate how opaque it is]? wouldn't that help with playing the game without having to turning the flames entirely off, as some tori textures are based on those flames?

tl;dr - Add a slider in the options that controls the opacity/transparency of all flame.

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.