Slower Than Light is going to have a feature set unlike any popularized 4X games. What makes it different?
Most 4X games use either square- or hex-based movement (e.g., Space Empires), Node-To-Node movement (e.g., Endless Space, Sins of a Solar Empire), or direct Cartesian movement (e.g., Stars!, Crusade in Space). These movement styles are simple and easy to understand, and they make sense in-universe given the energy levels the races in these universes are operating at. In Slower Than Light, though, everything that exists in space move in an orbit around other objects. The range and speed of a spacecraft is less determined by the efficiency and power of its engines, but rather what its options are for placing itself into a favorable orbit that will take it to its destination.
Signal propagation plays a major role in Slower Than Light. Since information cannot be conveyed faster than the speed of light, communications coming in from more than a light day away will be subject to time delay, which will affect the reports and status that a player sees. If an event happens in Alpha Centauri, for example, it will be over four years before a message reaches Earth, and for Earth to send a response will be another four, totaling well over eight years for Alpha Centauri to get a reaction.
Threats in the later game will be dominated by this mechanic, and will increase the importance of NPC (or PC — see Multiplayer) governors of these far-flung colonies. While a strong and proactive governor will often nip problems in the bud, that same governor might decide that the Homeworld isn’t worth the trouble and schism the empire. On the other hand, a governor who runs everything past Earth before acting could easily let a problem fester out of control while waiting for a response…
Slower Than Light is a worldbuilding game as much as a 4X game, and that shows through in the Multiplayer. Multiplayer STL can work in two ways: in Traditional Mode, each player starts out with their own homeworld on map, and sets out to carve their own empire out of the stars.
In Legacy Mode, however, a player playing a single-player (or multiplayer) game invites another player to take the role of one of his NPC governors, and now that player takes control of a distant colony and begins executing policy, whether following the lead of the inviting player or not.
Another innovation Slower Than Light will introduce is the concept of “Light-Cone Turns”, which allow simultaneous and Play-By-Email play in the same game. Because of the absolute speed limit of information and material between stars, once a player has submitted a turn (by default, a turn is a day in STL), another player can take take turns ahead of that player until a message sent that turn might’ve reach their world. The practical upshot of which is that if you take submit a turn on Earth on January 1st 3,000 (game time, of course), my character 5 lightyears away can continue submitting turns until January 1st 3,005 until the game will stop me to wait for you to take another turn.
NOTE: It is unclear if balancing and testing of Multiplayer will make the planned October release.
Another unique element of STL’s engine is implementation of Special Relativity, and its affects on spacecraft moving at different speeds. Most of my readers probably know that as you approach the speed of light, time slows down for you. STL handles the reasonably complicated mathematics to calculate how those effects propagate the in-game objects, to make sure fuel consumption, life support usage, NPC aging, and message propagation all function properly to give you the best approximation of relativistic space travel we possibly can. For the moment, I’ll leave you to ponder alternate uses of special relativity’s dictates on a 4X game.