I don't believe it's legal to have a driver with no capabilities defined, but it's a bad idea regardless since most apps use capability-based selectors for devices, and so you'll run in to difficulties there. (Technically you can also select for devices by specific type/driver name, but I wouldn't normally do that. You can't do this in RM but could in a custom app.)
If you're writing a custom driver and don't have any attributes or commands that are good matches for the standard capabilities, you can use one of the "do nothing" capabilities,
capability "Actuator" or
capability "Sensor". Neither requires you to actually implement anything, though theoretically the former is intended for devices that accept commands (dimmers, switches, etc.) and the latter for devices that report attributes (most sensors)--but, again, it doesn't really technically matter. It just makes it possible to select the device by some capability, which is generally something you want for most apps.
Or if this is what you're asking instead: Rule Machine does not limit you to "standard" commands. There is an action called "Run custom action" that you can use to run any command (custom or even standard if you want to do it that way instead) on any device, as long as you can get the device selected. This selection is done by capability, so your driver does need to "implement" at least one.