The retro-gaming scene seems to be quite strong though and there are lots of replacement parts for other controllers: rubber dome pads, analogue sticks, switches, stick tops, button tops etc.
A custom gamepad could probably be made to use some of those ... but the company behind the original part might not like it.
Some home-made gamepads, and even the recent Neo Geo Mini's gamepad use analogue thumb-sticks instead of D-pad or digital stick. Then there is a circuit or software that converts the signals from analogue to digital.
The Neo Geo Mini's gamepad is widely disliked because of how inexact it is, not providing any tactile feedback. (and compared to the original Neo Geo CD's gamepad
which was digital and used microswitches ...)
On the other hand, unlike D-pads, diagonals feel the same as the cardinal directions, which I think is a drawback with D-pads.
My favourite Atari-style joystick is the handheld TAC-II
where the "restrictor gate" is
the switches. This makes every actuation distinct. The construction is very primitive: There is a conductive ball on the axis, which actuates by touching sheet-metal walls inside the joystick.
I have been thinking that maybe an analogue thumb-stick for digital input could be improved by adding an active
restrictor gate to it. But unlike the TAC-II, the gate would not need to sense direction, as that would be done by the analogue part of the stick. The gate would only need to sense on or off.
Instead of a gate as a hole around a shaft, make the gate as a touch-sensitive ring mounted to the PCB around the stick, and have the bottom edge of the hemispherical stick bottom strike it. Make the shape of the gate by shaping the hemisphere's edge.
Alternatively, make the gate as a shaped hole
from two parts of sheet metal sandwiched with an insulator in-between. Make the shaft be conductive (there are stick tops of metal) and use a capacitive sensor circuit connected to the sheet metal parts.