Custom length power/data cables like SATA/SAS with various connectors. The biggest obstacle would be that you'd need to test them, but you could start doing so with second-hand hardware. Pair it with a nice website with instant pricing calculator, and I bet you will see business.
Low resistance across each of the pins in the finished connector is not enough for testing? That's how cable tester devices, like for CAT5, work afaik. Maybe add another test for ground (like add another multimeter that is connected to ground and make sure it's not being shorted to ground) but that's all you need for most regular dumb cables, I would think?
Of course, if the cable itself is also a device and the cable has a protocol of its own that it needs to speak (looking at USB-C here, I read these stupid things need chips now to indicate they can safely carry more power), then things get more complicated
