Death and Taxes
A starship, built for obital bombardment, exits warp right at the edge of the system. Slowly entering the system, it lazily assimes a simple parking orbit. The planetary governor quickly hails the ship, "Good, we've been expecting you, we've had trouble". The grey eyed captain looked annoyed, "Make sure you have rest then, and give us the coordinates". Later, in the rainsoaked streets a group of three figures, all seeming very well armed, kicked opened the door to a small house. A woman huddled in the corner with three children around her. The captain's face, once hard, immediately softened, for the first time showing fear.
A group was riding hard late into the night. A whole town was riding on this chase, and a whole country their mission. Bullets began to fire toward them, and the posse fired back. Catching those banits, and what they stole, was the only way to make sure that the county got its due. That and making sure a whole city was safe.
The city crested over the hill, and the knight raised a mailed glove to stop his companions. The city had already arrayed defenses against them and it would not do to go flying the king's banner in these days, at least in this rebel city. This wasn't going to be a good day, but it was their duty.
Nothing is certain in life except death and taxes, except, of course, for in RPGs. Death is cheap (well, hopefully not) and taxes rarely come (or are just handwaved away). The answer, of course, is that taxes are a great plot hook.
If the PCs are tax collectors, then there are few important things. The first, being that the PCs are not lawless, they work for a legitimate government, but without needing internal ranks. At the same time they also have reason to be wanderers without a fixed address. They also have a thousand reasons for combat and moral decisions.
Tax collection in a modern, or otherwise well connected society, of course, is really damn easy. It's just a matter of asking for it, and then getting it. In a more spread out society it's not as easy. An interstellar empire, a series of fiefs paying tribute to a lord, a set of towns spread out across a desert state, and of these things mean you can send players to all new places and settings, with different problems and dilemas.
A good way to make this work is to give the characters legitimate authority to exceed the power of the local government in matters of them needing to pay the tax (and therefore setup conflicts with said government), while also give them the mandate to deal with problematic people inside the local system (and therefore, as part of the conflict, can be ordered to attack people they don't want to). This dual mandate will mean you have conflicts with large local authority, smaller conflicts with bandits and other people who are stealing the taxes (banits robbed the collectors), but also people who can't play due to personal hardships (sick, children, bad mangement, blights). Throwing moral problems, and violent problems, and other issues together seems awesome.
This leads to an episodic game where PCs have a lot of personal power, but also a lot of restrictions. Lines to paint inside, but wonderful paintings to be made. I think this is a great and overlooked plot hook. I definitely need to see this occur.