As I've mentioned before, I've started work on a true sandbox board game. I don't think this is a genre that exists, so let's unpack what I mean by that:
A sandbox game gives a player a world that they can interact with, but does not prescribe how they do that or what they will do. However it's important that it be more than a simple lack of objectives - a sandbox will *suggest* objectives. Minecraft doesn't say "Build a huge skyscraper" or "Dig as deep as you can" but its design suggests that you might do these things. A good sandbox game should have things that it's apparent that they player could do, that are difficult to do and that would be fun to do. A great sandbox game will allow for a modicum of flexibility in how players do these things or whether they combine some of these goals or otherwise permit them to express some creativity.
A board game is trickier to define. By the definition given by BGG a sandbox board game is arguably impossible - a board game requires a win condition. However I think that several win conditions suggested by the game fulfils that requirement, if the mechanism by which players engage with it draws upon the strengths and conventions of board games. The rules should be prescriptive and interactions should be deterministic. As soon as the game contains "Describe your action, another player judges how likely it is to work and sets a difficulty" then what you really have is a roleplaying game with bells on. Actions should follow a strictly defined set of in game physics that does not at any point require player action to judge the outcome of. Except in so far as players might decide to attack you for your behaviour
In the game I'm working on players are vampires living in an area dominated by five great city-states. Each game represents a month of time, every time you put the game away and pull it out again fifty years go by.
Each time you start a new game the cities have new leaders, represented by a deck of behaviour cards. This'll make them focus their population of expansion or building or war each turn until the game is over. The population respond to these dictates simply, walking around the map picking up resources or attacking opponents. This can cause the cities to gain new buildings or advances or to become happier or more prosperous or to build unique buildings or to lose most of their population in a great war. Over the course of games their story unfolds and some elements of their fate are carried from game to game.
So where's the room for the player? Well unlike Wizard's Academy or 404 rather than having "Monster turn. Player turn. Monster turn..." the players go while the AI is being executed. Vampires are natural manipulators and you spend your resources during the cities (and their leaders and generals and workers) being activated to nudge how they behave. Perhaps deciding that a leader will call for war rather than building this turn. Or decided an army will attack one neighbour rather than another. Or if you work out that a given leader's deck is five war cards and you're a pacifist you could assassinate them and have a new leader replace them.
So what are the objectives? Well you could pick a city and decide you wanted to conquer the world. You could decide to try to maximise the happiness of all of the people in the land. You could try to push towards an exciting building with lots of perquisites. You could start developing the tools you'll need to kill one of the other players (The rules for attacking another vampire feature the phrase "you automatically fail" free times but that doesn't mean there aren't ways around that if you develop in the right directions over several games).
Additionally each game player have three events. These say things like "When three civilian tokens are killed in a turn look at rules paragraph 135 and do what it says" You could decide you want all of your events to happen. Or to discuss them with other players and cooperate to trigger the ones you think will be most beneficial (To who? Interesting to you as players? Good for your vampire? Good for the people of the game?) Some of these will open up new mechanics and new avenues for exploration - but without ever destroying or stickering a component so that the world can always regress to where it was if you break it badly enough. Still, that's getting ahead of myself. The list above seems like a diverse enough set of objectives to be interesting for a first game.
There are some *tough* design challenges in this game. Pacing the release of new things to keep the game fresh without introducing too much at once or diluting every major event/reveal by having nothing to go with it. Figuring balance issues so that playing to undermine another player is interesting and a tense struggle between those players without totally destroying the world, but that cooperating doesn't let you manipulate the game so thoroughly that you can do anything you want. Keeping the component count low enough that it doesn't provide enough to bludgeon an elephant to death with.
I've already made an entire prototype for round one and resigned it to the "failed" bin without ever printing and assembling it - sometimes it's immediately apparent when a version isn't going to function. The new set of core rules feels much more stable and more likely to go in the direction that I'd like, hopefully I can have a playable prototype by the end of next week's design session.