As experienced in our last few games, if the Axis get lucky on their activation die roll and bring either Turkey or Spain onto the Axis side, a close competitive game can quickly turn into a rout.
The addition of the German units, the extra PP and the opening of key strategic locations creates an immediate imbalance to the game. Triggering Spain opens up Gibraltar while triggering Turkey opens up access to the Black Sea, Soviet Caucasus and the Middle East.
I would propose a more graduated process triggering of Spain and Turkey. For example, when the Axis capture both Greece and Crete, instead of a 1 in 6 chance of Turkey joining the Axis, Turkey gives 1 PP to Germany. This is similar to getting the 2 PP from Sweden when capturing the two Norwegian territories.
This could be continued such that for each additional 1 in 6 chance of a die roll trigger, Spain or Turkey sends an additional PP to Germany. If they are already giving all their PP to Germany the next step would be the allow Axis naval units pass through the Bosporus in the case of Turkey or allowing Axis units to rail adjacent to Gibraltar. Any further triggers after that would bring Spain or Turkey completely into the war on the Axis side.