The vast majority of accessibility considerations in games must be evaluated on a per-game basis. By definition games must involve some degree of challenge, some set of necessary barriers, and what constitutes necessary/unnecessary is entirely dependent on the mechanic of that game. So what constitutes a reasonable set of considerations to aim for is also entirely dependent on the mechanic of that game.
There are however a small number of considerations that can reasonably apply across most if not all games. For example subtitling of all speech, using a minimum text size of 46px for subtitles and 32px for all other text, offering both voice and text chat.
Implementing a small number of core accessibility considerations as certification requirements would not only have a dramatic direct impact on the level of accessibility in games, it would also reduce developer costs through providing extra motivation for engine developers to develop engine level functionality to cater for it.
Sega Saturn certification requirements for mono/stereo audio toggle and full button remapping