Not sure if this would be too cheaty but basically if a mob gets within attack range the crosshair would change to a red color (or whatever color the player chooses)

This is a good suggestion. I don't think it is cheaty, since the vanilla attack indicator also shows up under those conditions.

Note on this: it should probably take the colour from the "targeted entity" crosshair, even if a weapon crosshair is displayed. Will have to see how to implement this the best way.

Finally solved this by the way of implementing #43.