If an enemy army occupies a road/trade route within your borders then any leader worth his weight will take notice when all the traffic suddenly stops or slows to a trickle without explanation. Theoretically it's balanced because while you know something must be out there, you don't know any details of what or exactly where. Last time I checked roads are automatic and not manually built like in civilization, preventing abuse. Your opponent can use the road's speed boost to his advantage but at the cost of possibly giving up their element of surprise. It also could be useful in psychologically harassing an opponent by setting off false flags of enemy presence. Nothing helps player to player relations like a good dose of healthy paranoia.
GUI graphical representation might be coloring a road yellow, red ect when it becomes interrupted.
Outpost: (inspiered from Starbase in Sins and the Ruined tower in LoTR where Frodo got stabed by the Nazgul)
-Outpost can only be built on roads (what do you expect). They can be upgrade by 2 way that I can imagine, first more traditional is a build upgrade your puchase for it that takes time to build and increases defence and so on. Second, more original way is to have it being reinforced with troops, when reinforcing the troops now become part of the outpost (you lose control of them) and the type of troops determines what type of upgrade the outpost gains. (type of upgrade is left to imagination and debate)
-Outpost have a small 360 zone of influence aroudn them that will slowdown any hostile forces traveling throught it. This zone is uncontestable (EI you can claim teh tiles unlest you conquer the outpost) and this zone can not contest teh zone of influence of other players, you will need to conquer the enemies influence via teh otehr means the game has. (Ei you can not build and outpost next to enemy teritory to steal the teritory. It's not a terrytory stealer.)
-Outpost should provide protection for your road vision magic for around double the distance of the zone of influence. (So your roads cannot be stolen right up to your front door.
-Outpost strenght? Dunno how strong these should be but they should not stand up to armies, maybe only small gangs and such. (IE much weakear then the SBs of SINS in tearm of firepower and defence)
It could be difficult to rationalize (sell) this idea. Maybe have the outpost simulate entrenched medevil traps and a population defense presence in the area?