A Coordinate Puzzler

Say the Vyde universe is expressed in a 2D coordinate system designed to faciliate indefinite expansion in all directions. Further assume that it's a multiplayer game, and one person in a two-person group wants to see an arrow on their screen pointing in the direction of the other. In what ways might the orientation of the arrow be calculated?

If the coordinate system were simply integers, that'd work fine. And fortunately there are specialized types (F# has BigInt) that facilitate arbitrarily large integers. But what if the system needed to be more complex, as hinted at in the last post? What is necessary to faciliate such an indicator in the game's interface?