https://www.gtplanet.net/forum/threads/rumor-has-it.388028/
Apparently Polyphony mentioned to several people just recently and it already got leaked
... there will be "rain" in the next update
My usual trustworthy source (so actually no rumor...), couldn't say if it was going to be dynamic or just "wet surface"...
I still believe dynamic isn't in the cards, so my guess is we'll see new tracks (looking at Nations and Manus timetable I expect 2...) with wet surface scenarios.
Apparently Polyphony mentioned to several people just recently and it already got leaked