Red Bulls headache is that Verstappen would want a co-driver who would be content being a very clear second seat. Perez did a good job of that in 21 and 22 before his form dropped off. Lawson, Tsunoda are their obvious choices, but would either of them be happy to be a "Barichello" for Red Bull?
Bottas might, Magnusson might, but Magnusson looks fairly content at this being his last year in F1, (at least that's how he comes across in interviews). Bottas I think doesn't have the drive to perform anymore, although it's hard to tell in that dreadful Sauber.
Do they promote a driver from F2? Maybe try buy out Colapinto from Williams? After seeing what happened to Gasly and Albon at Red Bull would Colapinto want to?
If it was me, I would promote Lawson but I suspect Lawson might be a bit more fiery than Horner wants alongside Verstappen. I enjoyed Lawsons battle with Alonso in Austen and then Perez in Mexico, but I'm guessing Horner didn't!