Just had someone on the GS forum remark that 2018 would be the 40th anniversary of the Prophet 5. Is that true?
Work on the Prophet-5 started in April 1977 and the first and only prototype was presented at the NAMM show in January 1978. So yes, at the time of next Winter NAMM it'll be 40 years since the public first met the Prophet-5.
Don't forget that the Prophet-6 was already a reimagined Prophet-5, though, so I wouldn't expect anything that's closer to the Prophet-5 than the Prophet-6 already is.
Dave also mentioned at the last Moogfest that although the instrument they're currently working on carries the working title "Prophet something" (I guess the additional description would reveal what it's about), they're ideally shooting for a different name once it's ready to be announced.
So given these two points, I don't think that the 40th anniversary of the Prophet-5 is anything to go by. We'd rather see a successor to the Prophet 12 or something else.