Nintendo are currently working on a SNES-style wireless controller for the Switch, according to documents filed with the US Federal Communications Commission (FCC) which also reveal a potential release date.
The filing, a legal requirement for any equipment using certain bands of the wireless communication frequencies in the United States, is partly covered by a confidentiality request. However the device, described as a " Wireless Game Device" using frequencies between 2.4 and 2.48GHz reserved for Bluetooth communications, does have a single publicly visible image under the label placement field of the application. As you can see above, that's a SNES controller alright.
While we can't tell much else about the SNES Bluetooth controller's inner workings thanks to the confidentiality request, we can tell it is for the Nintendo Switch as the FCC application includes the internal model number HAC-042. The Switch console is designated HAC-001 and any other Nintendo-developed hardware for the Switch includes the model signifier "HAC". The NES-style Bluetooth controllers were filed with the FCC under product code "HAC-033".
The confidentiality request could also give us a tentative release date as well, or at least an announcement date. The applicant at Nintendo who filed the FCC document requested short-term confidentiality to end on February 6, 2020. Another Nintendo FCC filing, for the Nintendo Switch Lite (product code HDH-001) has its short-term confidentiality set to end on September 21, 2019 – the day after the console launches worldwide.
Following that logic, this gives us a release date for these Nintendo Switch SNES Bluetoth controllers of February 5, 2020. And given that Nintendo released NES-style controllers alongside the launch of NES games on the Nintendo Switch Online service, this could potentially be a release window for SNES games on NSO as well.
Of course, this news of a Bluetooth SNES Controller for Nintendo Switch is all based on a confidential FCC filing at the moment, but the facts line up so we're fairly confident we'll see it become a reality at the start of next year.