Li Shenron
Legend
I would solve the issue with design in three ways:
- Place some requirements, both roleplaying and mechanical, on multiclassing. Emphasize the story of what it means. Make them find a mentor/college and study for at least a year...after all the party wizard studied for several years presumably, it is only fair. Make the player choose trade offs.
- Create a "Gish" class (call it whatever) to handle 50% of the multiclassers.
- Playtest the hell out of the multiclass rules, since you know there are players who will try to use them to break the game. The first step will probably be not to front-load classes and establish a relatively equal power level between classes on a level-by-level basis.
They are all reasonable ideas, although all have drawbacks too.
Requirements are damn hard to design sensibly. Narrative requirements perhaps even harder. Alignment is the prime example of an apparently simple and sensible narrative requirement, and yet it causes hatred every time it is mentioned! I am all in favor for narrative requirements, but I believe it's best to let the DM define them, instead of hard-code them into the game.
Mechanical requirements can be hard-coded, but IMHO they should be kept simple, and easy to modify. Generally speaking, mechanical requirements can prevent some abuse but will always also prevent some legit build and allow some other abuses. There is no solution, except acknowledging that again the DM is the only person who can keep everything in check. "Maximum N classes per PC" is my favourite example because it's so simple, it tends to treat every combo equally, but then it is immediately modifyable (on individual PCs) by the DM if the group notices it's too restrictive/forgiving for their tastes.