Button remapping is essential for many gamers who are not physically able to reach some areas of the controller, or find some areas of the controller difficult or painful to use.
This is something that should be addressed at a game level. And in previous console generations it has been a certification requirement – no SEGA Saturn game was permitted to launch unless it had full remapping.
However, if in-game remapping is not a certification requirement, including system level remapping can provide a safety net. Ideally allowing full remap of all buttons, including threshold based remap between analogue and digital controls, and allowing mappings to be set on a per-game basis.
It is only a safety net though; it is far better to include at a game level. Game level means better findability (remap is predominantly used by gamers who have no accessibility needs, who are less likely to intuitively think to look in a system level accessibility menu), for flexibility (e.g. separate maps for different classes, as in Overwatch, or for games that have multiple control setups, such as driving and walking in GTA), for the ability for in-game prompts to update to reflect mapping preferences, and for the option of remapping to affect only gameplay rather than menus.
The existing XB1/PS4 remapping has resulted in some developers, even a whole publisher, thinking that remapping is no longer a developer concern. This is harmful, so efforts should be taken to make it clear to developers that system remap is not a substitute for in-game remap, and encouraged to implement remapping themselves. Xbox already consistently give this message to developers.
A test screen would be a useful feature for system level remapping, to allow gamers to test the results of the changes they are making without having to go in and out of a game.
Another useful feature would be to include accessibility options in the list of remap options, for example being able to set up a paddle on the Xbox Elite controller to turn text-to-speech on/off.