If reaching Rakua is not the focus of the next arc, due to specific circumstances caused by turn of events, then I'd imagine the next story adventure may perhaps revolve around a treasure hunt arc for specific artifacts that would enable passage into Rakua.
If a treasure hunting arc is the next phase of the adventure, this could allow the story to introduce the "Treasures of Ruin Legendaries", especially since Chien-Pao is seemingly a popular pokemon design. Also, since Lucius seemingly visited Area Zero prior to going to Rakau, perhaps the Hidden Treasure of Area Zero story may somewhat tie in to the narrative, which could include Miraidon and Koraidon respectively, along with Arven's parents (Professor Sada and Turo).
If horizon were to introduce a possible timeskip, which I would assume could be a one or two month timespan, I'd imagine the story to reveal Liko and Roy completing the gym challenge and Paldea League Assessment offscreen, Friede's departure from the RVT during the timeskip, and Dot working with the Paldea research team.
Then after the timeskip, I'd speculate the story to move forward with the RVT reaching Rakua with Friede's return, and then perhaps after things are resolved we get an Ace Tournament arc, Roy earning Rayquaza's acceptance as its trainer and then the series ending with the group going their separate ways.
This is my best guess so far.