I'd expect Orpheus content, probably Thermidor, at the end of the second season the way they introduced the idea of him in Calliope, to keep it in readers' minds. Season of Mists and Game of You are definitely harder to tie together than the first two books, but I wonder if it won't be done through the dream portions of Game of You, with more interaction between the skerry and the rest of the Dreaming. There's always the Alianora - Nada parallels to explore. (I may love Alianora to bits.)

Foxglove/Donna canonically knows Rose through the ill-fated Judy, so they could have kept in touch, and I could see her looking Rose up once Weird Things Happen. If Desire is the villain this time (via Nada and Alianora and Thessaly), they'd definitely keep on the side of the human realm, setting up their chess pieces while Dream is busy.