Do you mean alloy plate?
The biggest problem is the door seals press against the door card, rather than the door. So if you remove the card, or try to replace it with sheet alloy or similar, you'll have a huge gap all round the door, rain and noise will come in, and it'll be really ****e.
What are you trying to achieve? Once stripped of all the unneccesary guff the stock doorcards weigh ****** all, they're just light fibreboard.