First one's from me!!!
So, I've the trade system right now is unique but basic (you put thing up for sale/buy & people come & trade).
But what if a once in a while a trader would come to the based & say something like "I got robbed on my way here, be a lamb and destroy those raiders & I'll reward you!"???
Or traders could sometimes (for a price!) tell the player where to find a particular weapon/upgrade/resource cache (assuming those will be put in the game)?
I think both of these kinds of quests can compel the players to do more exploring/fighting thus adding to total game play hours.