The first game I designed, the original Black Powder \\ Red Earth FPS, used what would become later known/popularized in CoD Modern Warfare as a create-a-class system. I thought the idea of a custom load out, tuned down to the actual weapon systems and optics was an idea that was long overdue. In the following years, Battlefield 3 used a version of this concept but reintroduced a role based class system where ranges of weapons were paired to the different classes. DICE did this, I assume, to throttle the power of certain classes in the hands of very skilled players.
When I began work on this Black Powder \\ Red Earth game I decided to focus on a role based unit system inspired directly by the skill sets required for a real-world targeted killing mission. The core roles were collapsed into an assaulter(responsible for breaching, area suppression, heavy fighting), a JTAC(Joint Terminal Attack Controller - essentially a drone controller and overwatch unit) and a Intelligence Officer(responsible for signals intelligence, site exploitation and interrogation).
I also decided that there would be a very limited fleet of weapons for these units. Currently, there is 1 player selectable weapon per class: a MidLength M4(improved over the SOPMOD carbine), a M1A and a MP7. These weapons were selected and configured to perform in these environments, each with trade offs similar to their real world analogues. IE an M1A is going to be able to shoot through walls and pretty much kill anything with one hit, but when faced with half a dozen Quds Force at close range, a player might do better to suppress with the M4 or MP7.
Part of this was driven by the fact that in a third person action game, minute details like optics, barrel length, vertical grips and slings, would not have a tangible or visible effect felt by the player like they do in a FPS. The other thing influencing this decision, was as a board game, I had found that producing a limited number of units with specific balances required some interesting player decisions. In a game with 3 classes, but only 2 units, players always had to make a call about which specific skills they wanted to employ on a specific raid. These choices could radically impact how a raid played out and made the game a lot more fun, to us anyway.
Designing the look of these characters was also a long process. Presently, there are 2 player units designed at the moment and almost a dozen enemy and non-combatant units. However, given that there are several classes of enemies in BPRE, each with distinct jobs, capabilities and skills, we might end up with only one skin per enemy class. As seen in games likes Company of Heroes or Syndicate, there's not a lot of detail for players to track in environments of the scale and complexity we are building, so friend/foe recognition is of paramount importance, especially when planning attacks.
As we add complexity to our environments and explore the combat system in the virtual world vs the tabletop, there is a constant stream new considerations to factor in.