Paul has said:
Istanbul New Airport opens as a Turkish Airlines hub on March 3, 2019, and to other airlines much later over the next few years. Check before you travel.
This is practically impossible. TK, other domestic and all other international airlines will be moving out to ISL (which will become IST by that D-day) on the almost same time-frame. There's no such possibility that other airlines may follow TK "much later" (by months or by years).
@ ISTFlyer: You can add the "Emirates Lounge" to your above list. Paul (in his article....) clearly mentions that there will be one to open sooner or later, its location is already signposted. Now I wonder about the OneWorld chaps. BA & QR will be the major participants to ISL/IST, and as they do have a lounge at Ataturk, there's no reason why they will not have one here!... Also TK already has two neighboring lounges in the domestic terminal (one is currently/temporarily) used for their crews.