It's possible that Universal could still get Ready Player One, due to Universal's past partnerships with Amblin Entertainment (Jurassic Park, Men in Black) and due to Universal Pictures minority stake in Amblin Partners. RPO isn't out of the equation just yet, even if most of WB is.
It's also possible that they can partner with Paramount Pictures again (they've done so before, with Transformers), to gain rights to the Mission Impossible series, Interstellar, World War Z, and certain Nickelodeon properties. They could also choose to make additional attractions based on Transformers if Bumblebee does well.
And lastly, there's Sony Pictures. They own the rights to the Jumanji series, James Bond (which was previously rumored to go into USF), Hotel Transylvania, and Goosebumps, along with other properties. This is an intriguing option.