I had an idea for an extremely difficult, but extremely rewarding bandit camp. It would be an old military outpost with some of its soldiers still alive. Hundreds of Army personnel use the base as a fortress, raiding the surrounding towns and survivor encampments for supplies. They have tanks, helicopters, artillery and heavy weapons. If you manage to take it over, your character’s followers will be set for life. Massive stockpiles of MREs, medical supplies, guns and ammunition, as well as all the tanks, helicopters, humvees and other military vehicles that were at the base will be yours, along with massive amounts of other loot as well. This would essentially be a “game win” condition, with you still being able to play afterwards.
The most dangerous enemies would be the NPCs commanding the tanks and helicopters. You would either need rocket launchers, heavy weaponry, high explosives, or your own armored vehicle to combat them. The NPC soldiers themselves would be equipped with standard loadouts, military clothes and armor, with rifles, machine guns, shotguns, and grenades, as well as artillery and rocket launchers. You would need your own army to combat them.
The NPCs themselves can form “raiding parties” that will patrol and/or loot the surrounding area, meaning a large portion of the base’s soldiers will be away from base at one time. They are openly hostile to all other NPCs and monsters, and will kill everything on sight. They will form a squad and travel from their base to their destination, killing and looting anything they see along the way. Once they arrive at their destination, they clear out and garrison the area, looting everything in sight. Once they fill up their trucks with loot, they drive back to their base to unload it. Everything they loot will be stored at their base, meaning if you take over the base, you get all the loot that was taken (but not used, if it was consumable.)
You could possibly destroy these raiding parties, thus lowering the overall defenses/power of the military remnant faction. Any soldiers you kill on patrol reduces the total number you need to fight to take over the base. If soldiers are being attacked by other survivors, they will radio for backup. The closest NPC squad in the area will start travelling towards them to assist. This will take place in the actual game world, meaning they won’t just “spawn” at the NPC’s location, they need to actually travel there. If there are no other squads nearby, or if the player is able to jam or otherwise disrupt their communications, no backup will arrive.
If you can learn or anticipate their route, either through simple observation, guessing, or hacking their comms/computer systems for intel, you can lay mines/explosives along their route to ambush and destroy them.
If they lose any squads to enemy activity, they will heighten their alert level and withdraw some of their squads to their base to defend it, the rest going to the area where the squad was lost and conduct a coordinated search for the killers. The Player (or NPC attacking the patrol) will need to leave the area to avoid being hunted down.
The more enemy squads you take down, the tougher they get. They might originally only start out with infantry and trucks, with some larger squads having Halftracks and APCs, but with every squad lost the squads start getting bigger. They will start including tanks, drones, helicopters, heavy weapons and power armor the more squads get wiped out, and will be more aggressive in hunting down other survivors.
There could possibly be a quest where you can get different factions to ally against the military remnants, launching a coordinated offensive against them and eventually attacking the main base. Success would be determined by how good the commanders are at planning. The player could be a commander and use a special UI to designate squads and assign orders. NPC commanders can also exist, and can be assigned their own men. They will assign missions based on the NPC’s personal history and skill, life if they were a military officer themsevles, or had experience in leadership positions, or if they otherwise had any sort of training or skill in commanding men, or had high enough charisma/intelligence to learn and improvise.