you could try something which is a blend of Planetary Empires and the BFG campaign, so that moving your forces into a System which is unoccupied results in an automatic win. But the problem will be at the start of the campaign there will be very little fighting as people manouvre for 'free' Systems and then towards the middle/end of the game there will be constant fighting.
If you describe what you are looking for a little more, we might be able to help. Like what amount of fighting and suchlike. But just on the brief description it sounds like you might be struggling to find something out of the box so to speak. It may need to be managed quite closely by an Organizer and made up on the fly to a large extent.
Another option is rather than having a campaign map, play the campaign using Scenarios. If someone isnt there for the game then they are considered to be in transit or suchlike. The winner of the scenario takes control of the System and gets to pick the target of the next phase (normally have three or four scenarios lines up and give them a choice of three). The winner, however goes into the next Phase at a slight disadvantage due to them having to leave garrison forces at the last System, securing their new territory. That way it prevents someone from steam rolling one system after another.