Here's my thought for a GA-oriented sim...
The Imperials have decided to test the Galactic Alliance's strength and their skill in combat. However, they don't wish to waste Imperial lives to do it. So through a series of intermediaries, they have given the planet Almania a hypervelocity cannon. The Almanians, playing right into the Imperial's hands, decided to use this new weapon to their advantage. Centering their defenses around it, they launched attacks on nearby worlds in order to frighten them into submission. So far it is working since none of the planets are willing to send their already battered forces to take on the battle-hardened Almanian forces and an anti-orbital cannon capable of gutting any ship they send. Elom and Troiken have already capitulated and Quermia is considering it. However, Ord Radama has contacted the Galactic Alliance and is requesting that they interfere and help out...
Basically the GA has to go in and put the Almanians in their place. And they have to be careful about it too, because they don't want to look like they're invading worlds. So they'll have to tread carefully. However, they do need to remove the HV cannon and bring the Almanian fleet to heel. Plus they need to do it without taking too many casualties.
As for rewards, the GA would get to do a modd for a ship. Plus if they play their cards right, they could get Almania as well. However, if the planet's defenders manage to take out at least 3 GA capital ships(w/the HV Cannon), then the IR would get a ship modd as well. This way everybody stands a chance to win something. This wouldn't spark off a war between the GA & IR because the Imps used a few intermediaries to make the transaction so they'd be able to deny any official involvment and the GA won't want to push it too far either.
Who ever would be in charge of commanding the Almanian forces would get to create a Black Market fleet equal to 70KM (Almania would be a Metropolis in the scenario and we'll say that Elom & Troiken-having capitulated-are both industrial). This will allow the GA to bring a full fleet into the vicinity and the battle will still be relatively fair.
And as for the HV Cannon, the GA won't be getting its grubby little mitts on it because either they'll destroy it, or the operators of the weapon will. It'll all depend on how it goes.
The idea is make this battle last for a few weeks, because I'm sure that whoever's commanding the GA forces will need to be creative in order to take down that weapon w/o significant losses. Plus the Almanian's will be wanting to put their newfound power to use so the GA will have to keep them busy, etc.