Kinda silly to be talking about 1B. Every position has its unique skillsets. For example, what about turning double plays? If the metrics primarily focus on the one fielding the ball, I presume the play be the receiver at 2B does not get any credit? Most often, the receiver at 2B is the one that makes the more difficult play.
So the primary WAR adjustments are park and defensive position. Are there any other key adjustments? I am sure there are quite a few others that are less significant.