The deck system could play out in three different ways but all the ways are extremely long winded in MFB, but could potentially be very quick in burst seeing as how quick battles are. I can also see how it could be hard to enforce.
The three main ways that we played were:
"Triple show": Where you would build in private then show all three beyblades to your opponent at once but then put them in a miscelaneous order in your deck box. You would then play your opponent and you won best 2 out of 3 total rounds (or best 2 out of 3 matches).
This could be made competitively fair by doing a:
"Build for build": where one person builds a combo, and then shows, then the next person builds the next one, and shows. You would go back and forth building combos that would either counter the other combos they have, or to strengthen your strategy. You couldn't build the same thing over and over however. You could apply other rules, such as for MFB, making it so you can't use metal wheel combinations twice, or using certain tips twice. Again it would be placed in the deck box based on whatever strategy you wanted. And again best 2 out of 3 rounds (or best 2 out of 3 matches). But this would be the longest method.
The last type was:
"Deck Box Lock": Where you would progress through a tournament, but you would only be allowed to use a maximum of three total combos. You wouldn't have to show all three. Any combo you play against an opponent is placed into the deck box, and you would be forced to use that combo again or out of the selection of three throughout the tournament.
I know these are all really long winded ways to play and I know it wouldn't be truly feasable to execute it, but maybe with burst since it's so fast, it could be done. Or maybe in smaller showings. But of course it's also hard to enforce all of these.
Adding more rules just makes it harder for people to get into it. And you would obviously want to cater to the largest scope of people, young and old. Also forcing three beyblades minimum also raises the bar in who can really compete. It's part of why any deck system hasn't been implemented unfortunately
.