Hello slothman,
I have few ideas that you could maybe can add it to the mod.
First of all, HP. You could create pickups that would be objects not just a regular HP pickup, like food, drinks, medicine, bandages and others. Second of all, inventor. Imagine that you can pick up bandages, but you dont need it right at that moment, so you put bandages in your inventory or fuel canister (if you can make one of course). Third, missions. Make mini-missions, like to get gas by driving on checkpoint or get supplies from the grocery store, weapon crates, take over bases, helicopter crashsite.
Its just my ideas and i dont know how to make them real in scripting, so ill just leave it to you.
And sorry for my english, Im not very good at it.