If I had to guess, I would say it's yes, a PHEV Sienna is coming. We just don't know when. Maybe 2022 model year?
IMHO, it's just too easy, with all the engineering already done with the Rav4 Prime powertrain.
I can understand Toyota not wanting to say anything right now, them not wanting to give people any reason to not buy the new Sienna when it comes out.
I can also see a Highlander Prime, but again Toyota will not say anything right now to avoid the Osborne effect.
Osborne effect? Is that the Ozzy Osbourne effect that might cause buyers to bite the heads off live doves?
Seriously, the possibility of a PHEV Sienna and/or Venza will likely cause my wife to not buy a 2021 RAV4 Prime as planned. Her current 2012 Prius v hybrid is used 99% in town and we see no reason to buy a replacement that can't run in EV mode most of the time.
My wife doesn't like driving our 2014 Sienna due to its bulk but I think she would be willing to drive a 2021 Sienna if it was a plug-in hybrid and especially if it had a panoramic roof similar to her Prius. She loves that pano roof.
My wife was far more interested in the new Venza with pano roof as a replacement for her Prius but again it has to be a PHEV. There have been three identical high-spec previous generation Venzas in her extended family - all with pano roofs - and she liked them a lot. The new generation RAV4 hybrid isn't getting exactly stellar reviews and I'm not expecting the complaints to be addressed on the 2021 RAV4 Prime.
So ... I guess we'll wait a year or two on replacing the Prius which is at only 45,000 miles. Even if there is no acceptable PHEV Sienna or Venza, maybe something else will come along ... such as Tesla's new "million mile battery" which is supposed to be announced soon. Maybe I'll get her into a Tesla Model Y yet.
Dang, Toyota! Can't you give us exactly what we want just one time?