Silent Saturn,your proposal is interesting:
using a Class skill Trained: Advantage
using a Class skill Untrained: Normal
using a Cross-Class skill: Disadvantage
greatly increases the importance of class skill lists. I would recommend amending those lists positively rather than negatively in that case (ex: granting Fighter extra class skills like Acrobatics, Diplomacy, Heal, Perception, Sense Motive, etc, rather than compiling lists of 'anti-class skills')...
I think you've misread my post. I was actually making two distinct suggestions.
The first was that you are at a disadvantage when using an untrained skill. If it was a class skill, then that would negate the disadvantage, and you'd roll 1d20. Otherwise, you could put a rank into it to negate the disadvantage. Trained class skills only would get advantage.
The second was that each class gets "anti-class skills" at which he or she is at a disadvantage, regardless of ranks. A paladin with Greater Feint might have max ranks in Bluff and a beastly CHA bonus, but he'd still be taking the lower d20.