As I've toyed around with the idea of classes with less than four templates, I've had the idea in the back of my mind about using classes that are only a single template. There are a few ways that can be done, such as the Animal People made by Deus ex Parabola. The way I've been thinking about using them however, is as a little surprise. Maybe you touched something you shouldn't have, or played around in a place where once was cast fell magics. Maybe something touched you while you were there, and now it's waiting inside until it's time for it to say hello.
Infected
Maybe it was that weird eyeless fish you ate last week, but you've started feeling a little funny. It's a bit harder to keep food down, your muscles always seem to be sore, and that's besides the chronic ticks you've started to develop. It almost feels like something's crawling under your skin at night, maybe you've started to realize something's infected you.
[A]: Parasitism, Flight Instinct
Parasitism
You are a body made host by a colony of parasitic worms, manipulating your muscles and actions.
At any time, as long as theirs's an unconscious body nearby, you can choose to move the colony to a new body. This is a horrific display, and will no doubt kill the current host if they're not already dead, but you get to reroll your Build score.
Flight Instinct
You can give a few helpful parasites to a friend in order to "Help a friend out". If the individual ever loses control of their body, the parasites can move them out of harm's way, usually by fleeing the location. For every week somebody spends with a parasite in them, roll an x-in-6 where x is the number of weeks they've had the parasites, if the result is less, make a note that they are Infected. Upon their next level up, they gain the Infected template. Parasites can usually be removed by selectively applying poisons to the body, assuming one knows they're present.
Charged
Maybe you got pricked by a weird needled tube in the bottom floor of the dungeon, or swallowed a mouthful of the salty green liquid in the big old vat. Whatever caused it, it's set your blood boiling, and you feel more charged up than ever.
[A]: Life-blood, Sulfur's Animus
Life-blood
Your blood runs green and much hotter than it used to. If let out of the body it'll give off a temperature about equal to a campfire for up to an hour. Your body gives off an ambient warmth about equivalent to a candle in distance and intensity.
Sulfur's Animus
Your new blood wasn't meant to be blood, it was an energy source for a star-killing super-weapon, until you stole it. You can focus the energy into a devastating blast which vaporizes most objects and structures immediately surrounding you. Each time you use this ability, you lose 1d3 inventory slots as your internals seize up and your body fails.
