The Earth system.
Each system would be linked together via jump holes or jump gates. To add a bit of difference (it is Coalition territory), how about using the Babylon jump gates?
1: A system with Pluto and Neptune in it. Pluto could have a station orbiting it, along with a docking ring taking you down to the planet. The planet could be seen as Toledo, snowy, yet with signs of life. Neptune could have a spaceport (a Freeport model if you will) above it, with a battleship nearby (custom or Liberty model).
2: A system with Uranus, along with a spacestation like "The Ring", which is located in Freelancer near New Berlin. To add a bit of exploration, how about a huge space cloud nearby (orange or something, with those explosive pockets of gas found in one of the Rheinland systems), littered with pirates or something.
3: A system with Jupiter, along with all its moons. Notable moons would be titan, which could have a planet loading sequence such as California Minor or something. Ice is the theme for it. Jupiter isn't landable, but the one in the Earth system mod is PERFECT, its amazingly huge. Above the planet could be a Freeport like station or one of those huge stations like Newark in the New York system. Also, a shipyard should be nearby. Pirate bases or enemy bases could be located in a cloud or small astroid field nearby.
4: The final system, in the Earth mod it contains a dense astroid field (like the real one located between Mars and Jupiter. At the end of the field (the jumphole\gate on the opposite side, with tradelanes through it) is Mars, with a three or four different shipyards orbiting, and several small stations nearby (astroids like the Lane Hacker\Corsair bases maybe?) or orbiting as well. The planet is small, but not too small. The planet landing sequence (you gotta land on Mars lol) could be the one for Planet Malta or Crete, something with caves but signs of civilization). Across the system, through Tradelane travel, is that big blue rock. The Earth in the Earth system mod is perfect for the model, but there's literally no signs of life. Earth is IT. Its the home of the Coalition. Orbiting Earth should be several different battleships, along with stations, Freeports, and shipyards (not all bunched together but spread out). The essiance is to make Earth look like the bigest place to be. There should be rounds of traffic to and from places, with several cargo and civilian ships coming and going. The nearby moon, with a landing sequence perhaps (what believable one could be done?), should have smaller stations and cargo placements, maybe a shipyard (one with battleships being built, like the shipyard in the New York system near the Battleship Missouri (I think). A quick tradelane jump away is Venus. Venus could sort of be the Coalition Fleet location. There could be shipyards, huge fighter patrols, and a huge station to act as sort of the HQ for the Fleet. What would be interesting is to have a dozen or so Battleships, all sitting in space, all dockable. The idea is to put them there for eye candy, not actual engagement. Sure, movable, fight-worthy battleships would be cool too. Finally, near Venus is Mercury. Mercury should pretty much be nothing, I mean, they wouldn't put a station there due to the proximity of the sun. One thing that could be here is a jump gate (maybe to the edge of the Earth system, Pluto. Another idea is to have a sort of central hub of gates, all leading to the systems of the Earth system (Jupiter, Uranus, Neptune). Nearby could be a final jumpgate to one of the systems in Sirius.
The backstory to this could be various things. One idea is to have that the Alliance finally tracked down the Alliance, and by secretly building various jump gates in out of the way locations (the badlands in New York for instance, or systems like Omicron Theta and Tau systems, places rarely visited), the Alliance has begun a full fledged invasion into Sirius. If you want to make a huge mod, have it so a system like Kusari or Rheinland have already fallen, and adjust those people's reactions to you accordingly (shoot you on site). The idea to entering the Earth system would be in a civilian vessel (Rhino, StarTracker, or whatever), however any Liberty, Bretonian, or Kusari vessels entering would be shot on sight, and it would adjust accordingly. This is just an idea for a huge mod, but the system itself to have would be cool, just to explore what Earth and the system would be like in...what, a thousand or so years in the future? Its just an idea, post your thoughts if this could be done.