Don't know if anyone has proposed this before.
A useful and strategic tool (especially in alliance maintenance) would be if nations would be allowed to provide military assistance to other nations. This could be in the shape of sending soldiers, tanks, ships, planes, etc. to the other nation to be included into their military. Obviously there should be limitations about how many one can send, for example, you can't send troops to a nation that doesn't have enough barracks to house them, etc.