How many of the planets in this system are habitable. You could have mysterious and dangerous Ancient installations on many or most of the uninhabitable bodies, derelict ships perhaps of the Relay Builders and many asteroids may be positively riddled with tunnels and caverns dug long before the various races were summoned by the Relay signals.

All of these could be defended by ancient robotic defenses, occupied by space orc raiders or there might be rival explorers looking to exploit the treasures of antiquity or survive terrors they've accidentally unleashed. The possibilities for dungeon crawls could be endless.

More habitable worlds could also harbor ancient installations and wreckage of their own or be good places for overland adventures. Perhaps you could have fusion-powered hot air balloons sailing between floating cities in the storm-ravaged atmosphere of a gas giant.

Look at all those new pictures of Ceres and Pluto and tell me you don't see whole worlds to map on mere asteroids and dwarf planets. Imagine a castle looming over Philae's landing site. This isn't necessarily a project you can fit into one map. You could do anything from an atlas with pages dedicated to each world in the system to a single map with the larger planets represented as textured balls and a brief notation of what's present on each body. Or you could start with the latter and work your way to the former.

In addition to what I've already mentioned, you could have acid seas, fierce alien jungles, deep frozen tunnels under planet-girdling glaciers, etc.

Look at all the moons and planets of the Solar System and then add fantastic elements. Not just fantastic. Steal ideas from conventional science fiction, westerns(ala Firefly), and horror genres. Perhaps something like Lovecraft's Mi-Go or Old Ones lurk or sleep under the ice of some outer planet or bake under the sands and stone of some inner planet.