Let's share some Ideas about some fun heists.
Here's a couple I've come up with:
1)A Train heist basically you either rob a train from the drug cartels/Gold/Weapons/Cars You could first drop a couple of people on the train to secure the area and then use a cargobob to retrieve the cargo.
2)High-jacking a truck carrying valuable items using a couple of cars to slow the truck down. This could be done either by blowing the back doors open or jumping onto the moving truck and taking out the driver.Here's the basic idea:
3) Stealing weapons from the Military base such as miniguns or some kind of nuclear weapons as seen in the story mode.