it takes place in a universe where consensus reality is true, and thus the scientific method fails as a way to understand the universe, since the laws of physics are determined by how many people believe in them.

I know nothing about the story/game you're talking about, but I think this is an important and common misconception, and fanfiction that addressed it directly might be worthwhile.

If it were an utterly chaotic universe, like what Brunner portrays in the Traveller in Black stories, then I might agree, but what you describe sounds instead like a universe with a regular and predictable relationship between people's beliefs and events in the world.

I'd love to see stories about a rigorous thinker in such a universe working out ways to exploit its ground rules.

I know this was over a year ago, but Avatar: The Last Airbender would actually be fairly easy. We could simply shoe-horn rationalist values onto the personalities of the four elements.

Fire: Tsuyoku Naritai and The Arrogance of Einstein: they are a proud, ambitious people, who know what they want, and push hard to get it, who know their strengths and dispense with cloaks of modesty. They are highly consequentialist, and are not at all risk averse, preferring 50-50 odds of glory or ruin to 100% chance of modest prosperity. They understand the value of inspiring and organizing large bodies of people to make things happen, but don’t really give a damn about the rationality of their underlings, making up powerful ideologies to get people to do what they want. This doesn't necessarily make them evil, but it puts them at risk. They also tend to be overly dismissive of “purely” epistemic rationality, and narrowly seek out only those truths that they now believe will help them with their goals. They have a problem with thinking they can calculate their way out of ethical injunctions. Thus, they’re vulnerable to fall-out from black swan bets and unknown unknowns, but when that’s not the case, they would be the master instrumental rationalists of the Avatar world.

Water: Flexibility, the right kind of humility, allowing the flow of evidence to push them toward the right conclusions. They tend to have few pre-commitments to particular beliefs, and are good at actually changing their mind but are often too passive, and too emotional. Sometimes they can allow their values to be pushed around by empirical evidence, bleeding together "is" and "ought" and becoming less confident in both. Some of them might have a problem with doubting too much – giving undue weight to improbable possibilities, or having too many reservations about a slam-dunk question. They have problems with perpetual uncertainty, and tend to be too risk-averse to take the large, ambitious gambles that regularly pay off for the Fire Nation. Because of all this uncertainty, they are uncomfortable with large, overarching systems, including ethical systems and decision theories. They tend to just follow empathy and intuition in their actions, and don’t stress about fitting their actions to a theory. They never cause spectacularly awful damage like the Fire Nation, but at the same time never create massive progress. They just flow forward.

Earth: People who have no problem calling a spade a spade, and are hands-down the least vulnerable to the problems of chronic uncertainty, undiscriminating skepticism, and questions like “Does reality even exist?” or “What is truth, truly?”. They don’t always rely on just naïve common sense, but they more than any other nation are aware that traditions, laws, and ethical injunctions evolved for a reason. They will always carefully examine Chesterton’s Fence before tearing it down, but if they do, they will not be half-assed about it. They have methodical, exhaustive processes in place to adopt and change their beliefs, which makes them slow-moving but very confident. They stay “close to the ground,” both epistemically and instrumentally – they don’t spend huge amounts of time discussing the Problem of Induction or Torture vs. Dust Specks. They are a no-nonsense, “see it with my own eyes” kind of people. This makes them very resistant to woo and superstition and belief in belief, but also very resistant to true beliefs that are very far mode or rely on obscure evidence or extensive philosophy.

Air: These are far mode thinkers, philosophers and mathematicians by trade. They have an unusual emotional detachment to the world and to themselves, which allows them more than any other element to recognize and accept the contradictions in their own mind. They have a tendency to always zoom out, to go meta, to ask “Why?”. They’re the ones in the Avatar world who would spend hours debating their world’s version of quantum mechanics, who would try to formalize Occam’s Razor, who would have very strong opinions on Torture vs. Dust Specks. They are obsessed with universality and general patterns – and with all the strange, far-out conclusions they seem to imply. However, they tend to forget reality checks and boring, on-the-ground data collection, and are more likely than anyone else to fall prey to the potential nuttiness of abstract reasoning. While their robust philosophical frameworks allow them to accept very strange and speculative probabilities, their general personalities make it all a game to them. They are unlikely to do much about their beliefs, which might be a good thing considering the aforementioned nuttiness. Arguably the most intellectual and least effective of the elements.

I haven't submitted it anywhere yet. I've asked some friends to read it first. That way if I'm one of those horrible fanfic writers who can't understand how bad they are, I can spare myself the humiliation.

Also: Nunnally is the rationalist. Lelouch is a bit more methodical as well.