There's been more than three fire-based Robot Masters:

Fire Man
Heat Man
Flame Man
Burner Man
Magma Man


I'd actually expect more of the lesser-used themes. Explosives are always interesting, to join Bomb Man and Napalm Man. Also, the flash theme would be neat to revisit, like Flash Man, Bright Man, and Centaur Man. The periodic table of elements also remains untapped, so we can still hope for the likes of Strontium Man, Rubidium Man, Carbon Man, Francium Man... Particle Man... Triangle Man...

Can you guess what weapon beats Particle Man?