Well, lets look at the history of Rohan here. Who have been allies, for however a short period of time, with Rohan? The Druedain(in the book), the Galadhrim(in the film), Gondor, and the Ents. Additionally, you can kind of count Grima and his traitors as a sub-faction. So if it HAS to be a sub-faction system, and the Ents, Lorien and the Druedain are ALL out of the running, that leaves us only with Gondor and the Traitors. I have already posted a traitor camp idea, which I still think has merit, so lets move on to something completely different and new.
Name of outpost: Gondor Military Outpost. Cost: 1000 resources. The outpost would be garrisonable by archers, and come with an arrow tower by default. It would generate a medium amount of resources, something like 60-80. It can recruit 3 new Gondor themed units, comes with an active ability, 2 upgrades for the outpost, and a final upgrade for the troops trained from the outpost.
Outpost Active Ability: Strongpoint of Gondor. Heals nearby friendly units passively and increases the armor of the outpost and all nearby friendly units by 100% for 1 minute, takes 3 minutes to recharge.
Troop Upgrade: Military Assistance Permit, cost 1000. The Steward gives the men stationed at the outpost full permission to serve the Lord of the Riddermark and aid his armies in combat, unlocking heavy battle armor for all 3 units from the Gondor Military Outpost, increasing their armor and health by 50% each, stacking with Heavy Armor for the Wardens and Messengers. The upgrade on the units would cost 250 per unit.
Outpost Upgrade 1: Outpost Garrison, cost 500. 10 Gondor Wardens patrol around the outpost at all times using the Slave System, and defend it against any enemies. This also increases the health of the outpost by 5o%
Outpost Upgrade 2: Vault, cost 800. A vault for information and personal belongings, as well as funds sent by Gondor, is installed in the outpost. Increases the income of the outpost to 150.
Gondor Messengers: Cost 100. Extremely light cavalry. They would serve for Rohan entirely as a scouting battalion who is poor in combat until upgraded. Must be upgraded with Military Assistance Permit to unlock the other 3 standard upgrades, giving the cavalry similar strength in late game to standard Rohirrim.
Formation: Pointman. This formation would place 1 man several feet in front and the rest in a line behind him, and increase the vision radius of the unit by 100% at the cost of 50% armor. These cavalry serve as rapid scouts and messengers, to take messages of utmost importance and urgency to the Steward of Gondor. The man who delivers the Red Arrow to Théoden is one of these messengers.
Gondor Wardens: Cost, 400. Heavy Infantry unit that can go into a shieldwall formation and be upgraded with Banner Carriers, Heavy Armor, Forged Blades and Military Assistance Permit. These men are sent to a small outpost or embassy of Gondor in Rohan's lands to man it against any attack. They serve as more leisurely messengers to inform the Steward of Gondor of any developments in Rohan that are of less importance and do not require the Messengers to deliver the news. They would fill Rohans current gap of heavy infantry.
Gondor Skirmishers: Cost, 500. Light archers who stealth in trees. Can be upgraded with Banner Carriers, Fire Arrows and Military Assistance Permit.
Passive Ability: Surprise Attack. When the archers are concealed by trees and break their cover to fire on the enemy, they do triple damage with their first attack.
These are the scouts and spies of Gondor, who often travel through Rohan to gather news of faraway lands for the Steward of Gondor. They would function as a glass-cannon style unit, dealing very heavy ranged damage while being very vulnerable to all forms of damage themselves. This is to prevent Rohan becoming an archer faction, as they would require careful micromanagement.
That's the best idea that I can come up with for a Gondor type outpost designed to help the Rohirrim. After all, we know that Gondor has marched to Rohans aid just as much as Rohan has ridden to Gondors aid. Its a mutually beneficial arrangement, most excellent in Middle Earth. I think this outpost would work quite well as a recruitment point for some new units to spice up Rohans gameplay, give them some scouting options, and finally generate a bit more cash to help with the economy. Finally, due to the cost of the units and the inability to discount them, combined with the cost of the upgrades and the fact they MUST be recruited from an outpost, requiring map control, I don't believe this will break the gameplay or turn Rohan into an Infantry faction. This is just to fill some holes in their gameplay.