[andrego]

I've got a crazy alternative suggestion based, ridiculously enough, on playing Lego Harry Potter. Rather than assigning players at the time the controller is plugged in, assign player by listening for controllers after the game has launched. The first controller button touched upon game launch is "Player 1." The second controller with a touched button then becomes "Player 2." And so on. This may not be possible given how libretro cores work, but it would dramatically simplify everything, since the only time you really need to differentiate players is during game play. Possibly at launch you could have virtual controllers, so cores don't get insane, then as each controller has a button touched, that controller replaces the virtual controller invisibly.

edit: I think this would probably be the single easiest way for users, as it cuts out basically all decision making on their parts. They don't have to adjust things in a UI, because they never see a UI. It may not be as easy for developers though.

This - definitely this. A very user-friendly way to do it that gets the job done very quickly, easily, and 'stays out of the way'. I think users of modern consoles would feel at home with this approach as well. Games like Borderlands 2 and Diablo III work this way as well, for example. Hopefully it is feasible to take this approach in the code as well.