optional suggestion: to make the mission more exciting, make the illegal truck's and/or trailer's location known to all cops either anywhere on the map or larger proximity than normal. if innocent civilians can know the location of a securicar why not let police know location of illegal cargo?
some suggestions for illegal cargo items
-drugs (weed, cocaine, etc)
-counterfeit money
-real money but acquired illegally (dirty money)
-illegal weapons (firearms, explosives)
If you want to deliver illegal items there is already a smuggler mission but you use vans instead. You mean this could work the same way but with a truck and a trailer attached and police will only be notified once you enter the current active city with the illegal stuff. Not sure if it will serve any more good since there is already a kind of similar way of doing just that.
trailer can haul more cargo than a van so you'll get bigger pay. driving van is different than driving truck with trailer. this mission would make the cops the attackers and robbers defenders, the opposite of securicar missions.
i think it would be good in terms of gameplay variety so people don't get bored from doing same mission.