At the end of the game, you'd remember not just the script the game designers wrote, but how you and your brother fought the Mana Beast together. Brigandine could have supported multiple players, for instance. While this feature would have required a split-screen mode or taking turns on the strategic map, it would have added a new dimension to the game. Final Fantasy Tactics is a better example. Because the battle system treats enemies the same as human-controlled characters, with equipment and skills just like the heroes', the AI controlling a team of fighters could simply have been replaced with input from controller 2 to add a huge amount of replay value to the game.
Designers can learn from a few games which had multiplayer features added as an afterthought.
Go to the Options screen in Final Fantasy VI sometime, and you'll find an option to let controller 2 direct some of the characters in battle. Because player 1 normally has full control over the party, all FFVI 's multiplayer option does is take control away from player 1 and leave him issuing verbal orders to player 2. In contrast, look at the Sonic the Hedgehog action games. Normally Tails mindlessly follows Sonic, but with this undocumented feature he can collect rings and attack enemies. In the old NES game 8 Eyes , a second player could control the hero's hawk and fly to places the hero couldn't.