Bearing in mind that we've still got 3 sub-headliners to announce (assuming Years & Years aren't Sunday sub) and a potential A38 headliner.
If I had to place a bet on anyone who I thought was going to get added at this stage, it would have to be Billie. Looking at the spread of acts throughout the week, it's clear they've loaded to Wednesday & Thursday and Monday & Tuesday to try and encourage people to buy week long passes. However, they've neglected the Friday-Saturday-Sunday pretty badly in comparison to the other days so far(although I personally really like the Sunday line up, but the Saturday is super poor, Friday is good just not many acts added yet).
Therefore the logical move seems to be to add Billie as either Sunday sub, or 3rd down on the main stage on Saturday, to try and boost ticket sales, as her name alone would shift a lot of tickets, and I can tell that from the sheer amount of people complaining on instagram that she's not on the line up and how many likes those complaints have been getting (I know it's not relative to people who actually buy tickets as the people on instagram like to complain anyway- but it just puts into perspective how much of a marketable name she's become).
Aside from that, we have Wednesday sub & A38 headliner to announce and Friday sub. Seeing as the Wednesday is already sold out, it seems like they would be less likely now to have a really strong sub for the Wednesday. I could definitely see someone like A$AP Rocky subbing Martin Garrix.
For the Wednesday sub, Foals would be nice, as they are promoting their new albums and it would be cool to see them return to Sziget, and Wednesday A38 headliner I would preferably like someone like Bloc Party/Editors/Marina to be added.
I think it's really quite difficult to tell at this stage who the last minute additions are likely to be but we should definitely have at least 2-3 "big" names added at this point. I've always said as well that I feel like there's a lack of female acts at the top of the bill compared to last year (last year there was Zara Larsson, Wolf Alice, Dua Lipa & Lana Del Ray) whereas this year it's only really Tove Lo & Florence up at the top of the bill, so once again Billie Eilish would definitely fit in well in that respect too.