We can't replace them like for like so it's futile to even try. It'll be a decade before we get another chance to sign a player on that level again.
My priority for next summer would be the same regardless of whether they're staying or not: Sign a central midfielder. We have to sort out the midfield for real now. Every week I have to strain myself to not punch my tv when Xhaka's oblivious face appears on the screen. We have to get a new player in, preferrably two. My pick would be Jorginho. He's top notch, plays at the base of the midfield, strong shades of Arteta except ACL injuries haven't robbed him of all pace. I'd also take Fabinho. I'd get Nainggolan in addition to one of those two.
After that I'd sign a defender, a proper one. Maybe we can't attract the best attacking talent, but we can build a strong spine again that is hard to break. You have to start somewhere, and this feels like a particularly good end to do it in a summer where you're going to lose your best attacking players.
I'd go for the best attacking player we can find after that, but I think it's going to be a much of muchness. Even the likes of Lemar are apparently out of our reach. The real opportunity will be to wait and see if we can steal away some gifted player who's misfiring at their current club.