I've been going back and forth between a sector of space with multiple systems and a single system with numerous inhabited planets and moons for the StarSlinger.
The sector of space allows for interstellar travel and potentially more nonhuman species. It's spacial footprint is huge - virtually infinite. Jump drive, hyper drive, warp drive, jump gates - you need to find a way to significantly exceed the speed of light, or go around the speed of light, to make this work.
A single planetary star system, with maybe a baker's dozen of habitable planets and moons, limits the sandbox but still allows for night infinite space, especially as ships will be traveling at less than the speed of light.
Both allow for sandbox play while the single planetary star system allows for more focus, or at least that's how I'm viewing it.
So, after long thought, I'm going with a single planetary star system.
Three planets, along with two habitable moons, will lie in the temperate belt (yep, I'm mapping this in my head like a globe of the earth) Mostly earth like with some variations amongst the three worlds, these are the core worlds. Three super powers of different values and strengths.
Two inner planets (one with a habitable moon) will default to more arid conditions, especially around the equator.
Three outer planets with one habitable moon. Temperate around the equator, traveling any distance from the equator leads to severe arctic conditions rather quickly. The moon itself is barely hospitable to life.
The three core planets are attempting to colonize the inner and outer planets, for resources and military influence. Homesteading is common place, but with the exception of military bases and the occasional high tech urban center, most colonists are doing what they can do to get by. Self sufficient and for the most part, anti government - all government for a large part. The colonists aren't drawing lines between the powers, they are drawing a line around themselves.
With three powers vying for influence and secret treaties the norm, rarely is there open conflict near any of the core worlds. Too much to risk for to little in return. Instead, the colonies are much like pieces on a chess board, occasionally sacrificed to strengthen a position elsewhere. They know the role they play and none are happy with it.
Banking is independent of these governments and can be considered a fourth power unto themselves. It is headquartered on the moon that orbits one of the three core worlds and is responsible for the transplanetary monetary credit system. It has it's own planetary defenses and authorizes the Star Marshals with their power to arrest on all worlds (Law enforcement whose jurisdiction ends at it planet's borders would be the Planetary Sheriffs
Smuggling between planetary powers is a way of life for many traders, as taxes on goods can change in the time it takes to traverse between the two ends of the trip.
(all for now)