Balrogs
I think balrogs were originally Melkors
maia.
They are describes as
spirits of fire and their job was to help to mould the world in the beginning. I think they represent the destructive force that is essentially part of any creative work. When you create something new you have to cause a destruction for
the old.
So balrogs were Melkors maia. What does that tell us about Melkors special area of ability?
Fire is not an element - it is a phenomena.
I think he was supposed to be the
vala to govern the dynamics between the elements (water, air and earth/
Ulmo,
Manwe and
Aule). Thus he was in a position to rule the ballance of the elements.
He must have had a part of the power over each one, mainly a dynamic power: he could freeze water into glaciers or melt the earth in a vulcano.
So
Melkor was supposed to be the judge between Manwe's, Ulmo's and Aule's regimes. He instead wanted to be the dictator over them and the three had to make a multilateral arragement without Melkor.