AZ Gov Doug Ducey will appoint replacement. With his re-election coming up, I'm thinking he'll make his decision carefully.

http://www.smobserved.com/story/2018/07/01/news/john-mccain-to-resign-senate-seat-on-the-4th-of-july-says-aide-cindy-mccain-offered-his-seat/3517.html