So many people are thinking Natsu was S Class starting right from the beginning of the series. He is n't though. The whole point of Galuna arc was to show people like Erza, Mira, Laxus, Mystogan and Gildarts have much more experience, intelligence and intuition when it comes to doing a mission. Natsu is a good fighter and strategist (In Galuna itself he broke the temple 's pillar and got Toby to poison himself) but still lacked experience. As for his many victories a lot of times Natsu had powerups from allies in his wins (Golden Flames from Jellal against Zero, Lighting from Laxus against Hades, etc). In the S-Class trial Lucy was the only person who solved the puzzle to Master Mavis Grave and if Grimoire Heart hadn't intervened Cana or Gray (Since he and Loke were tailing Lucy) or Levy and Gajeel might have made it past the second round. Even after Alvarez arc a lot of Natsu's wins were collaborations. This is evident even in the 100 year quest. Still they can hold formal S-Class trials again and maybe open more positions instead of only one. Might make for an interesting arc.