What makes for a commercially-viable tabletop RPG is now a well-known fact, and this fact also applies to the use of a tabletop RPG to create and run a successful and entertaining campaign with that tabletop RPG. Building a campaign around the conceit that the players assume the role of a group of mercenaries engaging in their bloody trade, for whatever reason, hits all of those linked elements dead-center of the bullseye. It's popular enough that some assume that it is the default campaign paradigm.
Let us be clear about what a mercenary is: a combatant-for-hire, often specializing in a specific method of operation, who renders services related to combat or warfare as an independent contractor to clients in return for compensation on a contractual basis. A magic-user can be a mercenary, as can a psychic, or anyone else whose Occupational Character Class does not explicitly call itself a mercenary; it is the case, however, that most of the mercenary operators will be Men-At-Arms and often pursue the mercenary career as part of a family business (or something similar) and that will be reflected in the formal name of a character's O.C.C. (i.e. this is where your Headhunters, Glitter Boys, etc. come from) simply because warfare is a mundane and ordinary suite of skills that anyone can pursue and master.
So, banish from your mind any one-to-one association of mercenaries with specific Occupations.
Now, that said, a successful mercenary campaign has a few elements that cannot be ignored:
- An environment (milieu) full of potential armed conflict. The specific region wherein you intend to run your campaign must have two or more distinct groups that are already into conflict, but has yet to break out into any serious combat encounters; no side has yet to do violence to another, but the tension preceding such a watershed event is present. Mercenaries thrive on war, and without potential clients there is no need for mercenaries to maintain a presence. Furthermore, these conflicts must be solvable, and solvable on a lasting--if not permanent--basis.
- An environment wherein the conflicting powers lack the means to handle all of their dirty work themselves. This constraint not only is one where material resources, specifically manpower, is too scarce to handle all of the conflicts before them. It also includes political means, so as to deflect or negate undesirable political or economic consequences for acting under their own flag. Mercenaries operate in the liminal spaces wherein these conditions exist; when these conditions fade, so does the space for mercenaries.
- An environment wherein the mercenaries are free to avoid bothersome entanglements, in favor of pursuing their own goals. Mercenaries have their own ambitions, and that means that players playing mercenaries should also come to the table with a long-term goal in mind. They will be wise to stick to that plan, which means actively and regularly reviewing relationships with clients; they need to know when to cut someone off, when to change allegiances, when to quit the field altogether, and when to go all-in. Mercenaries are businessmen, and playing a campaign cannot avoid dealing in the business side of being a mercenary.
- An environment wherein there are other objectives to pursue that are not actors in the environment per se, but may contribute to it in some fashion. This is where your treasure troves, caches of pre-Rifts artifacts, your ley lines and nexus points (especially ones where a reliable or controlable rift exists), or other site that possesses strategic or logistical significance in the region comes into play. If there is no contract on offer, or one worth doing, something like this will be pursued by a savvy mercenary group seeking additional revenue streams or capital for fueling company operations. Furthermore, companies seeking to transition into socio-political players want to find sites like this to take, hold, and build a power base upon. They are ripe opportunities for adventure, and therefore for conflict; have plenty of them in your game.