Okay, to add what I said before (and to bump this thread) I noticed something today that clinches the Combustion Man/Shiva connection (in fact, I can't believe I didn't notice it before).
Check out this picture of CM's third eye:

Notice the three parallel, horizontal lines that are "behind" the eye, so to speak.
These three lines are very close to Shiva's symbol. Notice the three horizontal lines on his forehead here:

This mark is also worn by Shaivite devotees:

Now, Shiva is not the only Hindu god alluded to in ATLA, in realtion to the fire nation. A fire nation duel (the type of duel in which Zuko was scarred) is called an "Agni Kai." Agni, of course, being the Hindu god of fire.
Now I find this subtle use of Hindu "imagry" (for lack of a better term) in relation to the FN very interesting when opposed to the distinctly Buddhist flavor of the Airbenders.
I better stop here. One of my majors is Religious Studies (emphasis on eastern religions) so I could geek out over this stuff all day. 