I am running a campaign where my players have a set arch-enemy. A person dedicated to the same goal as the players, but does everything in his power to stop the PCs getting to it first. He would even go as far as to destroy what they all seek, if he can not have it.
This style of two party play makes for interesting development of all the characters involved, and makes the players feel important when a person are devoting huge amounts of time just to stop them. The downside to this scenario is that I feel as if the players have gotten comfortable with the fact that they know who is their adversary and what to expect from them.
The other side of the coin would be to scrap the idea of an arch-enemy and make the players combat the environment and whatever might be inhabiting it. Instead of a mad doctor cackling with glee at their every mistake, just make the players feel as if they are climbing the world around them. Varied sized obstacles on the way made to slow them down, and make the game about the players.
I am currently cooking up an arch-enemy for my players to encounter for the first time. Should I just keep them facing off against an ever changing world, or will that just make them tired and quell the sense off achievement you get from finally defeating the Big-Baddie?