http://www.1-hp.org/2016/10/28/esports-health-it-starts-with-ergonomics-and-posture/
Split keyboards are popular. One maker of such devices explains the ergonomics of their layout.
https://gaming.kinesis-ergo.com/ergonomics/
Logitech's G13 programmable keypad is worth every penny. Razer and other manufacturers also make keypads for gaming. Check the "Customers Who Viewed This Viewed These Products" section on this page to find more.
https://www.amazon.com/Logitech-G13-Programmable-Gameboard-Display/dp/B001NEK2GE
Some gaming mice could be useful as well as they are programmable. The products are legion. Do your research to find one that will work for what you need. Speaking of mice, there are some geared toward those who are left-handed now. Again, do your research. If possible, handle before buying. These come in all sizes and some will fit in your hand better than others.
https://lefthandedgamingmice.xyz
Although the company is no longer in business and I cannot find any of their stock left online, the XtendPlay Ergonomic Adapter sounds like a great idea. It might be possible to rig up something similar if you are handy.
http://www.coolthings.com/xtendplay-looks-comfy/
https://kotaku.com/5716759/review-xtendplay-works-for-extended-gaming-sessions
SquidGrip for the xBox One and PS4 are meant to increase grip over long play sessions but they might be of help.
https://www.amazon.com/Wicked-Grips-High-Performance-Controller-Grips-Xbox/dp/B076GZMTDH/ref=dp_ob_title_vg
https://www.amazon.com/PS4-Squid-Grip-Playstation-NEW/dp/B01H66EQT6/ref=sr_1_4?s=videogames&ie=UTF8&qid=1519248214&sr=1-4&keywords=squid+grip+ps4
These PS4 and xBox One controllers are made for the one-handed gamer.
https://www.evilcontrollers.com/store/accessible-controllers.html
One Switch in the UK had a ton of online information, from news and articles to a shop selling disability standard switches and controllers (second link below). They also list switch games. Check the Menu to explore.
http://www.oneswitch.org.uk
http://www.oneswitch.org.uk/shop.php
Broadened Horizons sells (among other things) game controllers and parts.
http://www.broadenedhorizons.com/products/accessible-game-controls/game-converters?p=2
Quad Stick sells custom stick controllers for quadriplegics.
http://www.quadstick.com/shop/
Naughty Dog's video talks about how the company tackled accessibility issues in Uncharted 4.
http://game-accessibility.com/2016/06/06/uncharted-4-thiefs-end-new-adventure-video-game-accessibility/
There are organizations dedicated to helping more severely disabled gamers. If you can contribute, please do.
http://www.ablegamers.org
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