[FairyKing]

Here's an idea, drawing heavily from Promare and trying not to rip off Rescue Fire too much:

"The blazing firefighter, Kaen Red!
The rescue firefighter, Kaen Pink!
The powerful firefighter, Kaen Orange!
The science firefighter, Kaen Yellow!
Hearts burning hotter than any flame! Shoboushi Sentai Kaenger!" (Firefighter Sentai Flame-ger)

In the near future, humans began to combust into flaming monsters called Burningen, seemingly at random. In response, an enigmatic scientist assembled an elite team of firefighters, the Kaengers, who use special weapons to extinguish Burningen and return them to normal. They also have to face the secret society Volcano, Burningen with control over their powers, and the tech company Fubuki, who seek to study Burningen for their own ends.

Instead of a traditional sixth ranger, both Volcano and Fubuki have their own extra heroes, who are sometimes allies and sometimes enemies to the Kaengers.

From Volcano, one who's mastered his Burningen powers enough to become an armored warrior instead of a monster:
"I'll roar like a wildfire! Burning Dragon!" (green)

From Fubuki, the first prototype of the corporation's attempt to make their own counterpart to the Kaenger system:
"Justice at absolute zero! Koriger Zero!" (blue)