Well, this would be somewhat possible, but what about the placement? How does it make any sense that I must decide to buy a Carrier 1+ rounds ahead, but I can decide if I'm actually building this carrier in England or in Australia instantly when I place it?
This can also be solved with the two-steps upgraded units; look at the Battleship of Total World War. There are no limits to the number of upgrades required.
I actually believe production and placement should be at the same time, as it makes no sense that you must decide what you are producing ahead, but you can decide where you are producing it just when you place it.