1. Make the evil enemy (e.g., aliens, robots, or whatever). They are technologically advanced and gunning for the Humans. They do not like Humans. Earth is five jumps away. The prize solar system is 10 jumps away and five jumps from Earth. The evil enemy knows Earth is five jumps away. The evil enemy knows the prize solar system is 10 jumps away. This makes searching faster.
2. Make Earth a multi-faction start with mineral boost. Make it a TN start. The evil enemy is five jumps away. The prize solar system is the opposite way and also five jumps away. The Earth factions have snapshots of the evil enemy and prize solar system as intel.
3. Make a prize solar system. Give it a prize planet or two, good ruins, massive mineral resources, some reasonably tough Precursors, and stock it with enough technology an Earth faction could match the evil enemy. It is five jumps from Earth. It is 10 jumps from the evil enemy. Make sure there are fewer prize planets than Earth factions.
Primary Win Condition: Make it to the prize solar system and defeat the evil enemy's attack. Be in control of a prize planet when the last of the evil enemy is destroyed or leaves the prize solar system. Evil enemy will launch one large attack. Evil enemy will probably scout the prize solar system first if the GM remembers to RP.
Secondary Win Condition: Avoid getting destroyed by the evil enemy for 30 years. This assumes you failed to make it to the prize solar system or the Earth factions that made it were defeated in the large attack. Killed means you lack the ability to expand. For example, mineral or fuel locked, out of Industry, etc.
GM Win Condition: Kill the Earth factions in less than 30 years. You only get one large attack against the prize system. You need to RP and be fair as you have full knowledge of what is happening. Can write the AARs from either the evil enemy or Earth faction perspectives.
Restrictions: Earth factions not allowed to settle the same body (e.g., moon, planet, or asteroid). Earth factions can take bodies from each other through war. Do not tell anyone the technology level of the evil enemy. Can flex the evil enemy's technology advantage, number of prize planets in the prize solar system, time restriction (i.e., 30 years), and distances between the three solar system to make it different each time. Make sure to tell everyone how far away the prize solar system is so they can find it more easily. Do not tell them how far away the evil enemy is, that is part of the fun. You can let the intermediary solar systems randomly generate or pre-define them. Maybe drop interesting stuff in between the three solar systems like nebulae, black holes, star swarms, etc. Basically, things that make the Earth factions work together to get away from the evil enemy in time to put up a fight. Makes it even better for when the Earth factions are forced to betray each other toward the end to try and claim the primary victory. Especially as they all knew from the beginning how many prize planets exist.