A while ago I made a SNES pad adapter for the C64/128 and then I patched some games to use it, I then also made 128 versions of them because why not. Here is dropzone converted to 128 - the pad adaption code cloud.cbm8bit.com/oziphantom/standalone128prg.prg sadly it doesn't enhance the game, no 2mhz in the borders etc
Awesome oziphantom! I remember rigging an (S)NES controller to my C128 some years ago. But like you say, existing software can't handle it... you need to re-write the game. Any game that uses more than 1 action button would benefit greatly from (S)NES controller.
I have thought about (and remember seeing on eBay) hardware adapters like this. You plug (S)NES controller into adapter, and plug adapter into C64/128 and you get immediate satisfaction (no re-writing of game required). Only problem is 99.9% of games can't detect more than 1 button, so all those extra NES buttons do nothing. A shame, but not your fault!
If Nintendo used a standard connector, I would offer to build/sell such adapters to the CBM community... but they use a proprietary connector that is almost impossible to source these days... (please correct me if I'm wrong!)