Thanks for the helpful feedback. My goal is not to make the game longer or add to the complexity. This is the only version that I own now, and I like the fact that maybe more people might play it.
I will go back to the idea that I had in the house rules.
Industrial Production Certificates
Total income for each country will be the IPC chart (not territories) x2 throughout the game.
For example, Germany starts the game with 24 IPCs, but it can only produce 4 units in Germany.
I believe that this will not add too much to the length of the game. Simply, players will now have an opportunity to buy higher priced units. So, Germany could buy 4 tanks on its first turn. More variety in purchasing could actually make the game more attractive to new and veteran players.