Restricting myself to 'civilized' encounters (meaning, not animals, monsters, or empty land formations) here's what I can come up with: Toll gate, or toll bridge. May be a legitimate government fine, may be a bunch of brawny idiots with an attitude. Travelling entertainer (bard, gleeman, minstrel, etc.) Travelling group of 'players' (like a small acting and entertainment company) or a small travelling circus. Small band of brigands, waiting to prey on the caravans you had mentioned. An escaped criminal, fleeing from a city justice system. The bounty hunter hired to collect the aforementioned criminal. A pair of young newlyweds, fleeing their powerful, but opposing families (see 'Romeo and Juliet syndrome')
Also remember that a major route between two large cities will probably be well travelled by caravans, as you mentioned, and there would probably be small villages and farming communities scattered through out. If the ground is not farmable, there will be inns in place of the farming villages. Probably one every eight hours of travel for a caravan, which might translate to two or three a day for a small, mobile, party on horseback. (That last is just a swag. It does sound like a lot of inns. But then, not too many are going to be great huge ones.)