Pirates are playing in the sandbox

This saturday, we concluded our first Pirates of the Spanish Main adventure. I used the adventure that was printed in the back of the book, added some things and made some minor changes to the characters. I also built up the Baron Pettigrew to become a recurring villain.
And instead of going to plan another adventure I will now prepare for a sandbox campaign, where the players fully decide what they want to do. I will of course always have some single-sheet adventures ready, but for the most time I will try to avoid “rail-roading” the players.
In my opinion the Spanish Main is perfectly suited for that kind of campaign. But hey, what do I mean when I talk about a sandbox campaign? I am talking about a open-ended campaign where the player’s decide the course of action not a pre-planned adventure. If the player’s want to go plundering, it’s fine with me, if the want to do trading, it’s their call.
As a GM I will set the background and once in a while drop in some hints on some things I’ve planned in the past. Just like the “sub-quests” in some computer games. But there’s no epic “rescue the world from utter destruction” plot that forces me to railroad the players to some climatic fight at the end. If they want to bring down the evil Baron Pettigrew, it’s their decision not mine. But I have some ideas on how the world will change around them even without them doing anything. After all, even the Spanish Main in the 17th century was not a static place.