adjustable tension springs and locking mechanism for varied stick cap types (Xbox Elite series 2 does most of this but uses magnetic caps which would interfere with the TMR sticks so ball bearing connections or other option would be preferable)
Adaptive haptic triggers (PS5) which can be toggled to hair trigger mode via switches (Xbox Elite series 2)
multi-touchpad on face (PS5)
analog face buttons (DualShock 2 controller had this but only a few games utilized this… the best example was the PS2 era Metal Gear Solid games)
customizable “per-button” color assignment / micro OLED or e-ink screens so button graphics can be swapped (PBTails new controller does the per button RGB color assignment)
USB-C / 4 wired connectivity + charging
baseplate contact-charging (PS5 controller has these so you can set them on charging docks)
hot swappable battery pack + AA battery holder pack or ability to not have a battery on at all when connected via USB-C (Xbox 360 controller had this)
swappable non-magnetic Zinc-alloy faceplates (PBTails new controller has these)
removable back triggers with dedicated button assignments (like the Steam Deck’s L4/5 and R4/5 buttons; not just cloned face buttons like Sony and XBox do)
integrated microphone with hardware toggle (PS5)
proper “separate keys” d-pad… not the mushy type
touch-sensitive surfaces for every button and stick (Meta / Oculus Quest controllers do this)
per-finger-joint touch sensitive grips for each finger segment (Valve’s VR controllers did this)
the ability to separate the halves of the controller so that each hand could hold one half independently and have them track similar to most standard VR controllers (think combining the switch controllers and Quest controllers)
NFC communication (Amiibo-stuff for example)
If any single controller did even half of this, they’d easily be the GOAT.