Hoy, Person, you rag! This suggestion is about captains that are not found in towers or camps or wherever they spawn, but they lead a troop of warriors around them, and they have orders to move to a different location. I'll divide this into three sections; Captains and Spawning, Troops with Captains, and Orders.
Captains and Spawning: In TTT, near the end of the book, we see Shagrat approaching Gorbag and they carry Frodo off to the tower of Cirith Ungol. Now Gorbag is a captain from Minas Morgul and he has been ordered to Cirith Ungol. But in the mod, captains just stay in their towers, and refuse to come down and go to war. Why is this? Don't captains move around as well? My solution to this is to rarely, say, maybe 5% of the time a new unit patrol (spawn) occurs, a captain and his troops (see below) spawn in.
Troops With Captains: Now a captain wouldn't go around with no bodyguards, would he? Seems like a good way for some tark-scum to bellycrawl up all snakelike and stab that pig in his back. So I believe that the troops that spawn in with the captain should follow him around, just like troops will follow you around if they've been hired. The troops that will spawn in with a captain will be random, but elite troops should be rarer.
Orders, you lubber: Captains don't go out of their towers and forts for nothing. There are orders echoing around with the big bosses that the cap'n has to move about here and there to sort out some spies or traitors. Captains should find the nearest structure within (at first) a 256 block radius (keep expanding if they don't find anything) because a captain better know where he's going after all and then start searching for the next one, or stay there. This process of moving and choosing can be repeated up to four times. An alternative is making the captains appear like traders, and they simply roam around.
Good idea, but if this is implemented then the chances of finding a captain in towers and camps should be lowered in order to make access to a stationary captain rarer. If done right, then whether or not you can get troops is less certain and players would be incentivized to carry coins with them to buy troops whenever they see a wandering captain instead.
Furthermore, since most servers have limits to how many units you can have hired at one time (Thus making the advantage of cheap units basically worthless), factions that have lots of cheap units could have more captains in order to represent the easier access to troops of said faction.