Strangely enough, I think you could actually model WoW and other MMOs here. You could design a set of "professions" that the players can improve at, integrate with the magic item creation system (in some cases) and have both work as non combat roles with some usefulness in combat. Smithing, leatherworking, engineering, use residiuum as the basis for an enchanter (one who gives temporary bonuses to items). We already have the alchemy system to provide a framework for this. A largely non combat secondary game of improving craft skills, collecting recipes and new techniques, and perhaps even engaging in the marketing of those skills could play a role in the game.
If that's the sort of thing you're looking for. Myself, I'm with Cadfan.
I would say the simplest way to do this if you really feel such things need some rules weight would be to have each character, at creation, pick a background skill or two. Make a short list, allow the PCs to make up some of their own. About anything can be a skill here. Assign it a primary ability, give the player a trained bonus, as long as its applicable, and add it to the character sheet. So they all have and improve in the adventuring skills through adventuring, but these secondary skills can improve through use, which encourages their use in the game. You could combine this with the idea of crafting professions above.
I'd avoid some of the gamist ways these craft professions actually play out in WoW. I'd make it more about learning new techniques, learning how a particular powerful item is crafted, setting up shop and collecting materials, RPing commission work.
Before 4e came out, there was a big thread in which people were hating on the excerpt that had been released on magic items and the D&D economy, with its 1/5th sell price. I defended the idea pretty strongly, as I think it works very well if the assumption is not magic stores on every corner. During the course of the discussion, I had the idea for a game in which the PCs play a group of adventuring magic item brokers. They would do commission work, travel the dangerous wilds to get to clients, make ample use of magical communication tools, maintain an extensive network of contacts across the realm, research legends of powerful lost items to find their likely resting places, and plumb ancient ruins in search of these items to, usually, sell. There would be a lot of resource management, as profit would be needed to maintain contacts, offices, gear, transportation and the like. I think this kind of game would work well with 4e and would work well with something like the craft professions above. Have each member pick a specialty in both crafting and magical knowledge. One guy is the leather worker and expert on ancient civilizations (history with a circumstance bonus for things relating to ancient cultures), another is the blacksmith with expertise in military history, another is the dungeoneernig expert and social face, etc. Think that would be a lot of fun actually, might work as an OLG.
