Provide a menu of controller functions that can be brought up and chosen from at any time – hit a shortcut, then be shown a selectable list of every direction/button on the controller. This enables access for people who are only able to operate a restricted number of inputs on the controller.
At one end of the scale, a game that relies heavily on every button, and a player who can use most of the inputs, apart from lacking the strength to click in L3 and R3. Hit the shortcut button, and choose L3/R3 from the list.
At the other end of the scale, someone who can only operate a single input, but through a combination of the controller assist menu and a switch scanning system (see later in this document), can still play games that have complex controls.
Ideally, to avoid unnecessary motor and cognitive demands, the list should be customisable, containing only the inputs which that player needs to have shortcut access to.
This can currently be done, but through a complex external system running through connected PCs. Operating system level would be far easier for gamers.