Would be awesome if QOTSA (and also the Smile) could get a second chance. Other potential headliners could be Nick Cave or the Cure who both played the festival before.
The Last Dinner party is also around those dates (though probably not as headliners).
From that list and not considering possible co-headlining slots, only Green Day, Eminem (never played in Portugal) and Depeche Mode could sell out a day at NOS Alive.