The problem is that a simple patch that would let every player use a standard controller instead of GBAs is that even if you could get that to work, you'd have to really edit the game in some extensive way because in multiplayer when each player enters a house or cave, they get a private view of where they are on their GBA's screen. In single player mode without a GBA, this screen pops up on top of the main screen. So the game would have to be altered where each player gets that same intrusive screen pop up over the main screen, and to make that screen smaller and in one distinct corner of the main screen so that if multiple people enter buildings at the same time, their subscreen doesn't cover other players' subscreens.
While I do believe a patch could feasibly be made to allow for each player to use a standard controller instead of a GBA, I'm not sure at all if it could ever be done to reasonably give each player a subscreen in a different quadrant of the main screen. Maybe if the game was decomped or something like that could such massive changes be programed into the game. But as things stand right now I wouldn't hold your breath on such an overhaul to the game.