Red Bull have had a taste of success- they’re not going to want to go backwards. When Webber retires, Red Bull will be looking for someone good- someone who can work with Vettel to win the Championship. If they go for a rookie, they’ll have to be pretty damn good. I think an established driver is more likely.
I don’t see Hamilton leaving McLaren- at least not for the time being. And Button seems pretty settled as well. Massa would be a possibility, but if Ferrari get rid of him, why would Red Bull want him? Kubica would be another contender, but of course that relies entirely on him returning to F1 at all.
Now, I know lots of people here will tell me to stop talking silly, but I still think Raikkonen will make a return at some point. And Red Bull would be the PERFECT place for him to go. For those of you who dismiss a Raikkonen return, I look forward to saying “I told you so” in a year or two time :)