You nailed the concern on this perfectly by describing the fact the Colecovision, Intellivision and Atari 5200 (CX52) have very similar controllers. What I really wish is that the three communities would get together and do a Kickstarter or GoFundMe to develop an improved, common controller that can be used with all three systems. I don't know what other consoles used a numeric keypad as part of the controller. It's fairly unique compared to all modern controllers made in the last 35 years. I think when NES came out in 1985, those type controllers started to disappear from production. By thinking in terms of a common controller for all three systems, it seems a lot more feasible to do than just going it alone for just one system.
My idea would be to make the controller flexible to serve all three consoles. I'm an Intellivision fan, so the numeric touchscreen could be flipped upside down (software config) for Colecovision and Atari 5200 fans whose controllers had the numeric keypad on the bottom (the three buttons above the CX52 joystick would have to be combined onto the 15-button touchscreen - a compromise). The four side buttons could be near the middle of the controller (another compromise). Both ends of the controller would have ports for connection to the homebrew console or PC with different cable adapters. Configuration software would be created to program the controller to immediately act as a duplicate of one of the three console's controllers. The directional disc itself could be configurable with attachments to closely represent each console.
