Maybe a mechanical tyrant in a steampunk - inspired setting. This tyrant has created an army of mechanical beasts and is trying to rule the city. Pyro, as a girl with fire powers, is the only one who can melt the metal parts of the beasts and stop the tyrant. The tyrant sees her as a threat to his rule and is constantly trying to eliminate her.
It could be an envious rival. Maybe there's another girl who is jealous of Pyro's unique abilities or charm. She constantly tries to undermine Pyro and create obstacles in her path.
I'm not entirely sure specifically as there could be many different 'princess guard archon visual novels'. Generally, it might involve a story about a guard protecting a princess in a world with archon - related elements, perhaps with elements of fantasy, adventure and character relationships.
A really interesting Pyro x Scout fanfic I've read is 'Flames and Speed'. This fic delves deep into how they might interact on a daily basis in the TF2 universe. It has some really cool action scenes as well as some heart - warming moments between them.
Probably the pyro pony itself is a main character. There might also be some forest animals or magical beings like a wise old owl or a mischievous fairy.
One common theme could be loyalty. Since it involves a princess guard, loyalty to the princess would likely be a central theme. Protecting the princess from various threats, whether they are political, magical or physical.
There is 'Archon's Quest'. This light novel is full of adventures. The world - building in it is quite fascinating, making the readers feel as if they are in the story. It follows the journey of the main character in an archon - filled world.
Yes, there might be. The 'Harry Potter' and 'TF2' universes are both quite popular, and fanfiction often combines different fandoms in creative ways. So it's very possible that someone has created a fanfiction where Harry Potter is related to the Pyro character in TF2.