[Suicide Junkie]

Dosen't that make it a bit too abstract, though?

There is no way to boost your defense of a certain map other than adding allies; if the other player is a bit better, they will almost always win since you start on even terms...

With the scenario, you could actually arrange stronger defenses for certain systems, and have persistent a persistent universe at the planetary level.
The invader could turn it into a raiding party by attacking only a few lightly defended planets, and then retreating, or make it a full scale invasion with troops and colony ships.

In order to simplify the job of the Moderators & admins, the scenario could be set up by the two conflicting players.
First, the game is started with the two or more players as regular empires.
An appointed leader for the defending side will generate the defender's colonies (predefined) and ship distributions (flexible).
The attacking side's leader will then get the savegame, and generate their ship distributions based on the declared forces from the RP.
Both sides will review the savegame, and once ready, the game will be uploaded as a scenario by an admin to PBW, whereupon the battle starts.

After the game is over, the admin tallies up the surviving planets and ships, and updates the RP as appropriate.