It might be more realistic to have plasma catch fire but is it better for game play? If ships need to worry about taking damage from venting plasma they'd probably just stop venting plasma.
STO is about my Liberated Borg Federation Captain with his Breen 1st Officer, Jem'Hadar Tactical Officer, Liberated Borg Engineering Officer, Android Ops Officer, Photonic Science Officer, Gorn Science Officer, and Reman Medical Officer jumping into their Jem'Hadar Carrier and flying off to do missions for the new Romulan Empire. But for some players allowing a T5 Connie to be used breaks the canon in the game.
Plasma is already getting a massive boost from the Romulan set, the Romulan weapons set, Embassy consoles, and other things. Adding an additional splash explode-y firey proc as well?? Not so much.
If it was a specifically a set bonus of some point, a three piece proc bonus for a Romulan anti-borg set (adds a 2.5% proc chance for plasma fire to ignite and cause additional kinetic explosion damage to target and all additional targets within a 3 km range.) THAT would be awesome.
Actually, that would be HECTIC on my current build...
An "eject warp plasma" you can detonate is currently the console power that comes with the assault cruiser refit in the form of "metreon gas canisters." It is, however, only able to equip to assault cruisers.
^This... Plasma is a state of matter. Fire IS Plasma. Plasma is not a fuel gas or something. It is not flammable it IS the FLAME.
"Some flames, such as the flame of a burning candle, are hot enough to have ionized gaseous components and can be considered plasma. This subject is, however, hotly debated."
Plasma used by starships in this game has been said to be at millions of degrees Kelvin, so you shouldn't be using flames for comparison as it is only partially in the state of plasma.
As for the OP - it would make more sense to force ejected warp plasma undergo fusion, but a simple explosion nearby wouldn't be enough, so just forget it.