The interesting question that I have never seen a Civil War game address as of yet is whether foreign intervention on the side of the Confederacy might have led to foreign intervention by others on the side of the Union. I am not talking necessarily foreign troops. But Prussia and Russia, rivals of France and Britain respectively, might have then felt free to provide aid to the Union.
Regarding Russia, as noted at
http://www.voltairenet.org/article169488.html
"...in the first two years of the war, when its outcome was still highly uncertain, the attitude of Russia was a potent factor in preventing Great Britain and France from adopting a policy of aggressive intervention....the proposed British-French interference promoted by Lord Russell, the Foreign Secretary, in October 1862 was deterred at this time mainly by the Russian attitude...Russell’s note to Palmerston conclud[ed] that Britain “ought not to move at present without Russia." The critical importance of Russian help in deterring the British and Napoleon III as well is borne out by a closer analysis...Seward thought that if the Anglo-French were to assail the Union, they would soon find themselves at war with Russia as well....The most dramatic gestures of cooperation between the Russian Empire and the United States came in the autumn of 1863, as the Laird rams crisis hung in the balance. On September 24, the Russian Baltic fleet began to arrive in New York harbor. On October 12, the Russian Far East fleet began to arrive in San Francisco. The Russians, judging that they were on the verge of war with Britain and France over the British-fomented Polish insurrection of 1863, had taken this measure to prevent their ships from being bottled up in their home ports by the superior British fleet. These ships were also the tokens of the vast Russian land armies that could be thrown in the scales on a number of fronts, including the northwest frontier of India; the British had long been worried about such an eventuality. In mid-July 1863, French Foreign Minister Droun de Lhuys was offering London the joint occupation of Poland by means of invasion. But the experience of the Confederate commerce raiders had graphically illustrated just how effective even a limited number of warships could be when they turned to commerce raiding, which is what the Russian naval commanders had been ordered to do in case of hostilities. The Russian admirals had also been told that, if the US and Russia were to find themselves at war with Britain and France, the Russian ships should place themselves under Lincoln’s command and operate in synergy with the US Navy against the common enemies. It is thus highly significant that the Russian ships were sent to the United States."
With regard to Prussia, like Russia they refused Anglo-French attempts to have them support an armistice. Although less likely to become militarily involved at sea (there was at this time no real Prussia navy) the Prussians and their German allies were a potent counterpoise to France on land in Europe. Given how many Germans were fighting for the Union cause, it is possible foreign intervention by Britain and France on the side of the CSA might have led to a stream of "volunteers" from the German states and Prussia. The main Prussian impact however, like Russia, would be as a deterrent to France and Britain.
ACW doesn't really take the prospects of a foreign counterweight to Britain and France. The USA, like the CSA, could muster foreign allies. Perhaps this is implicit in the force totals brought by intervention as it stands. But I would love to see a series of diplomatic and military assistance options open up to the Union AFTER intervention occurs. For example, if Russia enters the war on behalf of the Union, it would have a huge impact. Basically, it would provide options for the Union to bring an end to intervention by mobilizing its own allies. Instead, right now, once intervention occurs it is basically set for the rest of the game.