I think the general rule of thumb is 'if you think it's OP, it is'.

Acceptable agent power level = (experience * humor / target department) - bad idea quotient.

... Does that make sense? I think that almost makes sense.

Ahem. Anyway. Theoretically, there is no upper limit. I think the most powerful agents I know about are basically demigods. It's entirely possible.

BUT.

BUT. The thing is, there's not generally much point to it. So you have a character who can destroy his enemies with a glance or rewrite reality with a stray thought. So what? An action agent still has to go on missions, wear a disguise, watch the fic, write charges, and carry out the Duty like everyone else. An infrastructure agent is basically a 9-5 working joe; not much call for phenomenal cosmic power there.

The only reason to bother with a high-powered character is if it has some bearing on their characterization that will make them more interesting to read about. If it adds humor, so much the better.

One example, of course, is my Jenni. She's basically a demigod. She takes physical form because she feels like it and has a whole host of powers, including the aforementioned ability to rewrite reality, because she has access to the source code.

But she doesn't. She knows better—but she didn't used to. Her backstory, both meta and in-universe, is all about learning that throwing power around willy-nilly to solve your problems isn't a good idea, and can actually really annoy people. Plus, if you do that to other people, you're taking something away from them, and that's wrong. And at some point she also realized she doesn't have infinite power, so the more she uses, the more she tires out. So, despite having a very high power level, she also has great limitations. The only real legacy of her power is the lessons she's learned from the mistakes she made with it in the past.

Not that she doesn't give in to temptation every now and then, in which case she has Nume to scold her and the Flowers to slap a humiliating Sue-tracker on her. Fun times.

~Neshomeh

When I made Wtak and Smithers, I didn't give them innate abilities, but rather the capability to learn and/or master new skills fairly quickly- Wtak was constantly experimenting with different types of magic and Smithers with technology, and Smithers additionally often gained abilities from his disguises.

That way, their capabilities could adjust dynamically to the rough power level of the continuum and Sue they were hunting at any given time. Furthermore, while I can't think of a situation where this was actually broadly displayed, the idea was that they would go into 'fics with the minimum amount of force they were expecting to need, and then if things got difficult trot out the big guns on a case-by-case basis.

I'm aware of the ESAS, who by their very job description probably have a higher average power level than most other departments. I'd think about these guys separately from the rest of the PPC in this thought experiment.