Octopath Traveler 2 is in development. Not a rumour. The developers of the first game confirmed this in a Famitsu interview. The said it's coming to Switch. However they did not say it was Switch exclusive.
Source - https://twitter.com/Stealth40k/status/1107259297413648384
Source - https://twitter.com/Stealth40k/status/1107259297413648384