Thing about making decks 40 cards, is, you can't have a deck type anymore. You're pretty much going to have to use different cards that don't go together, or can't complete things. Especially with 5 of each card. Elements has many different simple deck types, and that's what's great about it. As there isn't a way to get out cards you need specifically, or many ways to draw cards period; you'll be stuck. Want a mono death poison deck? Sure, add in some Flesh Recluse. No, you can't keep the poison/arsenic/bonewall only.
Flying weapon decks? Forget it, they won't be of use with just 5 animate weapons, or 6 in a 40 card deck.
Life will also significantly fall. Life wants it's creatures out fast, (Frog, Cockatrice.) with only 10 of those in a 40 card deck, you need to take other strategies.
So on and so forth.
What you suggest is good for tournaments, it tests peoples skills in ways the game doesn't actually allow without being handicapped against other players. However, I don't think this should be done with the actual game. Tournaments make things like this fun, and in my opinion it should stay that way.