I'm working on an alternate ruleset for a tabletop pen-and-paper RPG (like D&D). Recently I've been toying the idea of abolishing a level system. However, I don't want to remove the ability for a player to multi-class. Are there any games which don't have a levelling system, yet allow multi-classing? How do they do it successfully?
I suppose that the simplest way to address this would be to make skill cost something (i.e. experience points) and define that a given "class" gives a discount (like, 50% off) to a given set of skills.
Example (numbers and skill names are totally invented, of course):
Then you can say:
And so on. Maybe you can also impose loss of some skills (which are actually state markers, or "traits" or whatever you want to call it) in some cases -- i.e., for some special Paladin powers you must have "Paladin" as a prerequisite, and this may be "lost" during game, and require atonement or other special circumstances to be gained (or regained).
You're looking for a system that has: - Some kind of advancement which is not by levels; - different classes, instead of a pool of options from which anybody can choose.
I know of some games that work this way (so yes, they do exist), the first example coming to my mind is Apocalypse World and most of its hacks.
In AW each booklet (let's call it a class... it's more of an archetype) has a series of moves the player can purchase by spending experience. One option is to choose a move from a different booklet but you can do it a limited number of times and you can't do it at character creation.
As veritascitor points out, most games without a levelling system don't have a strict class system either. But many of them, like Shadowrun, have archetypes, essentially bundles of skills and abilities that players are expected (but not required!) to take together. You can get a dual-archetype character by just taking some skills from each of the archetypes you are interested in.
Looking at Shadowrun, you could build a decker(hacker in later versions)/street-samurai character. You would just select both combat abilities and computer abilities and then purchase both computer gear and combat oriented cyberware.
Such a character would lag behind a specialized street-sam in combat and lag behind a specialized decker/hacker in cyberspace, but it would be be able to play both roles. And it probably should lag behind, a generalist normally should not be as good as a specialist within their field.