There is no denying that marines did play a very important part in the PTO and it would be great to see them in as an option. I have never believed that out-right denying the player the option to do something was a good move for game design. However, Marines run into the problem with giving us the option of “you CAN buy marines and deploy them in huge stacks……buy WHY would you want to?” Marines are great in terms of flavor and for personal choice, however they die just as easy as Mech infantry, coast as much, defend as well, but have none of the extra movement that makes the mech worth the 4IPC coast. Also, the rules state that marines only attack on a 2 or less when conducting an amphibious assault meaning that if your fighting overland all you have are slightly discoloured and more expensive infantry, that die just as easy. So really, your paying the extra IPC for what works out to be a one trick poney. Not saying I dont like the idea, but I can see them leaving it out of the final product.
Including Marines would also put the game on the slippery slope of “well if you are going to include elite troops for America, why not include them for everyone?”. Again, this isnt a bad idea in and of itself, however, it might add more complications to the game. You would need to come up with different special abalities depending on what each nation would have. Also, in regards to Germany, their elite unit would likely be the SS and I believe that including them in the game would add a rather dark element given the history of these formations and what some of them did in the actual war. It adds something to the game that is left out by leaving the infantry as just genaric troops.
Again, I dont think its a bad idea as a house rule, or even an optional rule in the last offical rules update, I just dont think it should be a staple of the game