Would honestly be shocked though if the Saturday headline was a male band e.g. Tame Impala/The National/RCHP/Coldplay.
Only way I could see this happening is if there is a female co-headline with Garrix, e.g. Ellie G coheadlining with Garrix.
Otherwise, I think the Saturday headliner will be a female artist, similarly as Lana Del Ray was the last headliner to be announced last year.
Ariana seems feasible to me and it would only make sense that they booked the majority of the headliners at the same time, as they would have a wider selection of bands with more open schedules and also costs being cheaper to book the artists in advance. Yes, Ariana will be a lot more expensive to book over the last few months with the release of her new album, however if they booked her around the same time as Ed Sheeran was announced, she would have been a lot cheaper and is certainly a feasible booking.
There is a reason Sziget has changed and is now booking Post Malone, Ed Sheeran, 1975 etc and it's because they know that's what is going to sell the tickets at the end of the day and if they can shift more tickets than they ever have before, then in their eyes its the "best" line up so far, regardless of whether one individual likes it or not.
Post had a huge crowd at Reading last year and has now been bumped up to a co-headliner there as well, so Sziget booking him does not seem like the worst decision business wise, although he is not to my personal tastes(although would much rather have him over A$AP or Cardi B).
I honestly have no clue who we will get next however the 2nd female headline seems needed to me, despite Sziget saying they will not aim for a 50-50 gender balance like Primavera, they are aiming to promote diversity and therefore they will want their line up to look reasonably balanced between different genders on the whole. So far we have 53 artists announced in total, and so far only 11 of these have female performers in them(if I have counted correctly). A couple more female artists lower down on the bill as well as a 2nd female headliner is the direction I think they will be heading in from the line up announced so far. Could definitely see Marina make a comeback lower down on the bill.
If it is another female headliner, and not Ellie/Ariana, I'm not sure who would be feasible. They have had Pink/Rhianna/Sia before however these don't seem like such relevant bookings anymore. Ellie is about to release a new album and Ariana has just released a new album, so these two seem the strongest headline options in my opinion.