I've been thinking about class structure in RPGs. I mean classes like "warrior, mage, rogue", obviously.
Classes grow out of the kinds of resources and mini-conflicts that arise from the game's challenges. If games have similar challenges, they result in similar classes. D&D 4th is a notably refined version of this, where the classes are descended from super-clear tactical roles (controller, defender, leader and striker). But all games with classes have the same fundamental idea: every class addresses a particular opportunity or challenge within the game's framework.
This changes based on the framework in question. In early D&D, for example, you had clerics and thieves: both classes addressed the less combat-centric parts of the game. In modern D&D, these have been slowly co-opted into combat roles as the game has become more combat-centric.
Most modern games with classes revolve around a specific kind of challenge which I call "open combat". Basically, everything revolves around damage: giving and receiving and controlling it. The four roles in D&D 4th ed can be easily restated within in this framework. A defender excels at absorbing damage, a striker at dealing it, and a leader at increasing everyone's capabilities to do those things. A controller is probably better thought of as two classes - one which deals AOE/specialty damage and one which interferes with the enemy's ability to deal damage.
Some of you are probably holding up a hand and going "wait, that's not quite right...", but please don't get sidetracked by the particulars: I'm using D&D as an easy example, I'm not trying to analyze every nuance of a particular system. I'm saying that in general, classes exist specifically to deal with tactical challenges presented by the game rules, and D&D's classes are a good example of that. The thousand of other games which also revolve around dealing and receiving damage in open combat also divvy up classes in much the same way.
However, that's not the only kind of challenge which is available.
For example, in a game where range is an incredibly important factor and damage less so, you start to divvy up classes based on their optimal range, speed, and ability to interact with cover and terrain. This is significantly more limited in D&D, even using miniatures, because the damage dealt is still more important than the range it is dealt at. In military wargames the classes are often more delineated by their range and maneuverability rather than by health and damage characteristics. You have grenadiers, tanks, snipers, riflemen, short-range infantry for urban zones, and so on.
In fact, you could divvy it up the same way as we did before, only with "range and mobility" rather than "damage given and received".
A defender excels at surviving attacks from range - infantry, crawling along under cover. A striker excels at dealing damage at range - snipers, tanks, artillery. A leader enhances everyone's range and mobility - scouts, ATVs, command vehicles, etc. Controllers are either A) anti-tank/grenadier units or B) engineer corps to dig trenches and set up bridges and such.
Of course, this ignores things like air support, because I didn't include that kind of challenge in my theoretical game. By creating gameplay elements, you alter the nature of the classes required to cover them: if I added aerospace control, I now have to not only add in the classes for air-on-air challenges, but also the classes for gluing air and ground together (air infantry transport, anti-air ground vehicles, bombers, etc).
The more diverse your gameplay becomes, the more classes and the more specialized those classes will be. Hence the boringness of playing an old-style cleric or rogue: their specialties were outside of the "fun" part of the game, so they spent a lot of time just hanging around being bad at the fun stuff.
So... that's my design thought of the day. What sort of challenges will you base your game around? What sort of classes will be required to facilitate the strategy of those challenges? How fun is it to use those classes and manage the tactics required?