I honestly thing we should go back to 1e/2e style multiclassing and make it cost a feat. Because the 3e experience chart progression doesn't level off you avoid the problem of multiclass characters catching up and only being a level behind. By the time the party is 20th level, your multiclassed character is 14/14 level which is a bit better than being 10/10. I thought about giving a 20% cost break for favored class which would make the multiclassed character 16/16 by the time everyone is 20th level.