At the end of tonight's GOT stream on Twitch (via JACK DANGER), they were mentioning the announcement on Thursday and a couple of them said that it would be a "most excellent announcement". OBVIOUSLY that's a reference to BILL & TED'S but maybe they were just f*ckin with us, not sure.
I'm leaning BILL & TED's for now, especially since they're filming the next film, as we speak: https://www.joblo.com/movie-news/the-wyld-stallyns-are-back-in-action-in-bill-ted-face-the-music-set-photos