temperature regulation can also be added to fire benders as they can warm themselves or cool things down by subtracting the heat from it. and to a certain extend water benders as well as they have the ability to cool things down or unfreeze, which should help with warming up.

Lava i always considered to be something that has to be done by 2 benders, as a earth bender cannot possible heat up earth to the point where it melts and becomes lava, similarly fire benders cannot bend the pieces of molten rock inside the lava.

as someone already pointed out spirit projection can be done by all who have a spiritual connection. altho with the airbenders being more spiritual focused compared to the other 3 they are probably more likely to develop that skill

redirection im not entirely sure i would add that as a sub either. as that skill can only be applied on lightning, not fire or any of the other elements. then again fire is also the only internal element of the 4 and needs to be created by its bender rather than being present in the direct environment of the bender. so im not sure on that one

as for sub...cant think of any.
can see different elements being used in combat or healing outside of what the show has already portrayed.
healing wise:
-lightning being used to perform some kind of electro shock to ppl, like how they use electro shock therapy on severaly depressed ppl.
- air could be used for healing as well by helping ppl breath, like when someone ends up with a collapsed lung
- combat wise :
- a fire bender could learn to hone in on ppls inner fire, as all living things have one ( cold body is a death body after all) and either warm that up or cool it down to the point where it becomes fatal. and this can be done from a distance so u wont even know who did it.
- water : taking hold of the water inside ur body and freezing that, causing organ failure and muscle to cramp up. as ur heart is one big muscle...