If you want to seriously cross the line with classes, go with
G.U.R.P.S., as it has no classes at all. No levels or experience, either - it runs a point system; you purchase skills, abilities, and stats with points; a certain number are given to a starting character (dependant on the world, campaign, and GM; usually about 100 points) to initially build the character; after that, they are awarded based on the adventures. You would need to have some explanation (background) for the character - but the (theoretically, anyway) for what skills, abilities, and spells you choose is based on the character concept (and limits of the campaign world, as set by the Game Master). If you want a fighter-style character with some lockpicking abilities, you can - just hold out some points from the fighter design to add the lockpicking skill, and note that your fighter had a misspent youth (where (s)he picked up the skill, of course). Or perhaps the character dabbled as an escape artist. As long as it fits the character concept, fits the campaign world, and won't unbalance the campaign, it's fine.