Thoughts on Firebending potential sub-techniques:
Plasmabending: How I'ld do it is have plasma being hard to make with direct bending (as in you need the Summer Solstice or Sozin's Comet), and specialist training is needed to control the Plasma. It can however melt almost anything and can easily expand the amount of plasma by heating and deionising material into more of the 4th state. This leads to a weapon that skill firebenders can use against others of their kind, if they're willing to carry the plasma producing kit. The style of Plasma bending is to gather it in a sphere about a metre infront of them and whip it out as needed or focus it to pierce walls and armour.
Lightning Strike: A form of Lightningbending in which small circles are made with the users wrists to build up and hold an electrical charge, which is released in a tap or poke to stun a target as if they were hit by an Electrified glove. Used by Fire Nation Police Officers in the Post-100 years war period.
The Black and White Flames: Actually two techniques in one. The White Flame is the ultimate expression of Fire Bending; Pure heat, power and energy unleashed in an incredibly bright and hot flame that most fire benders can not even hope to resist. The Black Flame is the ultimate understanding of Fire Bending: If fire is energy, can one simply absorb the world's energy? The Black Flame is cold, dark and quiet, taking in energy to charge up the user's internal reservoir and weaken their enemies by chilling them to the bone. However, the White Flame takes to much energy to use without knocking them out, and the Black Flame can take in too much energy, causing internal burns and even spontaneous combustion for the user. The ONLY safe way is to use both at the same time, taking in and releasing energy in a display of Yin-Yang mastery of Firebending: Both understanding and embodying it at the same time. Sozin destroy the memory of this art to make Firebending match his ideology, which is ironic since his ancestors invented the technique to reperfect firebending after the Sun Warrior fell.