I'll preface my remarks with the admission that I haven't played Warplan (though it looks interesting) so my suggestions may be bogus.
ORIGINAL: AlvaroSousa
One other change I am considering is also allow Italy to DOW in May 1940 and removing the Paris requirement.
Mussolini didn't join the war until the defeat of France was inevitable, so it would not be realistic for him to join the war when the defeat of France seemed even more remote (because of the all-in strategy).
I offer the following alternative for consideration ...
The Germans were slow to build up war production. The easy victory over France would have led to some complacency. If the campaign in France had been more difficult then they may have gotten more serious earlier. One way to counter the all-in strategy would be to advance the economy multiplier for the German economy (or similar) if the French campaign is long and difficult. This would allow any otherwise game-losing, excessive German losses to be replaced.
The obvious measure of "difficult" is the time of the fall of France, but tying an advance to the German economy to that puts the Allied player in a conflicted position of having to deliberately play badly so that France doesn't hold out too long. The maximum achieved size of the BEF above some suitable threshold might be a better measure. A larger BEF would be expected to lengthen the campaign. The Allied player decides the size of the BEF and hence the adjustment to the German economy. After that they are free to hold France as long as they can.
Players that choose a historically sized BEF (under the threshold) won't be affected. Players that go for the all-in strategy will find it less effective in the long run. The relationship between BEF size and German economy advancement could be chosen to be balanced (commensurate to the average German losses) or punishing to make the all-in strategy not worth pursuing (after all it would have been politically impossible, if not infeasible).
An important consideration is what constitutes the BEF. I initially thought of counting the British forces in metropolitan France, but that wouldn't prevent some gamey workarounds, like hiding the BEF in the Low Countries, invading northern Germany or Denmark, garrisoning French territories with British units so the French units could move to France, or ganging up on Italy. I think it would have to be all British land and air units not in Britain and British/Commonwealth controlled territories.
Ultimately, the all-in strategy would have been politically untenable so it would be better IMO if the game could capture the consequences somehow. I also suspect it would not have been possible to build up a large army in the time available by neglecting the navy and air force - just as a shipyard can't suddenly make tanks, an air training school can't suddenly churn out artillerymen.
I'm with Harrybanana in thinking that air power isn't as effective as it should be. The consequences of neglecting the air force should be greater. I expect neglecting shipping would have led to critical shortages in Britain which would be reflected in the effectiveness of the army and the morale of the nation. Churchill would likely be dumped as prime minister and Britain would have sued for peace. Having Britain drop out of the war if the BoA goes very badly would be a powerful incentive to not neglect the navy, and curtail the all-in strategy to a degree.
