Shouldn't combustion be there though? I mean even though there was only one bender for it, it's still a very special skill. Sandbending? It kinda makes sense, but Plantbending can be performed by any waterbender too. So it only makes sense to put back Sandbending or remove Plantbending. I also have an idea. Instead of the "T". We should try and find a arrow pointing down. It would look better.
Combustion was indeed a special technique, but as far as we know, there was only one person to ever perform it, and just as we don't make categories for a single thing, neither should we add a symbol for just one thing. Plantbending on the other hand is more widely used and although it can be used by every waterbender, every waterbender should at least be made aware that it is possible as Katara did not realize it before it was pointed out to her/
Personally I don't think it is worth to go through the trouble to find that symbol for something so trivial, but I won't stop you if you want to go look for it.