As it stands, SP is just a sandbox game without much of a storyline and thats completely fine. But why not add more exploration to the game?
My proposal: scattered around the map, add crashed vehicles, abandoned buildings, sinking ships, cargo containers, etc. that spawn in random locations, similar to convoys. Unlike convoys, however, they won't have an Air tp Ground target and they cant move on their own. A new mission could also be added where you rescue important cargo from a crashed cargo plane or a sinking ship and taking too long causes you to fail. Additionally, another mission could be made where you deliver a heavy cargo container to a short, remote runway to resupply a scientific expedition. We already have the Kraken and the Brown Pearl, so this idea isn't too far fetched.
In my opinion, this would add more excitement to the game and promote using parts like the magnet and winch, as well as making rescue and cargo vehicles.