In OTL, Spain stayed out of the war because they were still recovering from their own civil war. Could the Fascists to have come to power without causing as much damage? Maybe a more successful insurgency or even a democratic election?
If Spain did join the war, fighting for the Axis, how would that have changed things? I'm picturing a more difficult situation for England, and Americans becoming very distrustful of Spanish speakers. I also wonder if Spain would occupy part of France, freeing up German troops to fight elsewhere.
So the challenge is to put Spain in the Axis and discuss the consequences. The only rule is no ASB.