I was thinking about the dual professions in GW1. I thought of an easy way to implement a pseudo second profession by using the upgrade slots on weapons. A medallion or sigil of another profession could be attached to the weapon to make the weapon and skills of one profession usable on another. Sigil of the ranger, engineer, warrior, thief, etc.
The sigil or medallion could be an ascended item. It could even be limited to single-handed/off-hand weapons. I think it would be interesting to run a thief dagger or thief pistol skill off-handed on the engineer.