Personaly I don't see only a small benefit for Gameplay with other Factions.
The Infantery is pretty well balanced. Rebs. vs Imps - they have the same weapons only different vehicles.
A post civil war map with Imps fighting vs Imps would be IMHO the easiest and most interesting Way to add a Second Faction.
First it doesn't need new Moddels, so any mapper can build a map with this background
Second it gives a different kind of gameplay - remember both sides use the same models... this means you must be more cautionious when your'e fireing
...lot's of accidential Teamkilling, but this is "real" civil war 
The maps should aim for big, largescale battles, to make a different to the imps vs. rebs special operations.
The Rebs care about their soldiers, so they tend to fight with small forces in a critical Battle (like Endor), they hit an run or the are on the run (Mos Espa, Escape from Hoth)
The imps don't care about their Stormtroppers, they are willing to fight with any means and can afford large Forces.
Ideas for Maps:
Large Fleet engagement
- each side a static Stardestroyer, plenty of Carracks, Nebulons and Lancers, lots of fighters.
Battle ends if all Carracks / Nebulons / Lancers are destroyed, or the tickets run out.
Additional work:
-> Repair and Rearming points for Fighters under the SD
-> Repair function for the Carracks / Lancers / Nebulons.
-> Player controls for the Stardestroyers Guns. Possible a Spawnpoint at the bridge. Players spawn in the gun and switch seats to switcht between guns, players should get killed it guns is destroyed.
-> Repair Droid Bay on the SD. Like the Probe Droide on Endor (other Texture ?) or R2 Units, but faster. Imagine the Droids crawling above the SD, fixing the destroyed Guns...
-> new Background Textures, containing other Stardestroyers in Battle. Random Blasterfire (without damage) is coming from those SDs, like the Ion Cannons fire in "Escape from Hoth"
-> AI for the Guns of the SD an the Carracks / Lancers / Nebuloons, AI for fighters spwaning at spwanpoints outside the SD ("fighters from other SDs"), to keept the "large scale Battle" feeling even on Servers with few players.
-> Making the SD destroyable by adding destroyable parts like Engines, Reactor,....
Large CityBattle:
Two Imperial Factions fight about a Imperial Base.
Maplayout:
Large, with districts:
Barracks, Warehouses, SpacePorts, FuelDepots, Strongholds, a big Stronghold for one side, a other big Stronghold for the other side.
-gamemode: Push Double Assault, with many CPs, but each CP should bee connected to 2 other CPs (ot aviod the "Tree Village Syndrome")
-Flat
-no green, no trees, no natural soil
The Buildings from Endor
-Turbolaser Turrets all over the Map, somtimes arranged with the walls from Endor to small strongholds.
-Lager Stromngholds as HQs with Turbolasers, AT-ST statics, Walls and Buildings.
Some few Statics from BF 2142 and Northern Strike like the large Storage Tanks, Pipelines, Cranes, Railroads, Trains...
-at the warehouse, the Fuel Depot and the Spaceport
Landed Ships as static- from Lambda Shuttles and Bulkfreighters to Gallowfrees at the spaceport
Burning, destroyed Walkers, Speeders an TIEs from "Endor Strike Team" and "Encounter at Mos Entha" in the streets
Each side should have a Headquarter with Speeders, AT-STs and Probedroids (and in one case a waiting Lambda Static on a landing Plattform as last CP
)
Addtional Work:
-> AI for Troopers
-> Turbolasers firing in the Sky
-> Aircombat with TIEs, TIE Interceptors and TIE-Bombers
-> Tanks (large open Area for Tankbattles under the Big ships on the landing Field ?)
-> AT-ATs 
-> Backgroundtextures to keep the "Big City Feeling"
In both cases the maps should work without the "additional work".
CYNIC, n. A blackguard whose faulty vision sees things as they are, not as they ought to be. Hence the custom among the Scythians of plucking out a cynic’s eyes to improve his vision
