What do you guys think ultimately clenched it for the Extra Deck so that it had an upper max of 15 cards?
- Fusion-Material substitutes: it became more and more necessary to toss in restrictions for even basic fusion monsters due to the ease substitutes provided. Towards the end, there were nearly a dozen different monsters you could use in place of the tough stuff so you could whip out some real bad-ass monsters like Dragon Master Knight or Elemental Hero Shining Flare Wingman. Who needs the Ultimate Dragon when you can just toss in a swamp king instead?
- Super Polymerization: just have one of every Fusion monster you can get your hands on ready to go, and you can always fuse practically anything together, and at Trap card speed no less. Substitutes don't exactly help this either.
- Synchro monsters: it seems that while some do require particular Tuners to be summoned, most can operate with any old Tuner out. It's a level of flexibility unknown even to the most devoted Fusion deck.