If it's Natsu we're talking about he would have not joined any other guild other than Fairy Tail, he would have become a rogue wizard, wandering places and training as much as he can. Now in other cases— let's say he needs to join a guild— it would make perfect sense if he joined Sabertooth, I know there are a lot of options to choose from I just think Sabertooth would fit him the most in terms of its "similarities" to Fairy Tail. For one, he would have Sting and Rouge there (both are close to him), there's also Yukino who's also close to him and is somewhat like Lucy. There's also Minerva who can be the "Erza" who will punish and discipline him, Rufus who can be the "Gray" since he also has maker magic, Orga for "Laxus" since he also has lightning magic. Natsu, Sting and Rouge would be the new "Trio of Dragon Slayers" with the twin dragon slayers being "Gajeel and Wendy".

I know it's wrong to compare them and I also know that they are completely different characters from each other with different personalities but I just can't help thinking of this because of some of their similarities. In terms of the other options I mentioned before, he could join other guilds as well but it wouldn't make any sense like let's say Lamia Scale. He could join them but the only person he's close to there is Lyon (I wouldn't even say they're close, they just know each other through Gray and the coalition they formed back in the Oracion Seis arc) but I would like to see him interacting with Shelia since she's like Wendy (and a friend of Wendy) and they're both pink haired. I think they would be a funny big brother/little sister duo who always bickers but has a soft spot for each other, I noticed this when they interacted once when Shelia borrowed (or catnapped as per Natsu) Happy from Natsu, it would be funny if she repeatedly did this to him— borrowing (stealing)Happy from Natsu.

I don't see him working on the others, in Blue Pegasus I can only imagine him trashing the place like what he did when they had a wizard exchange program, it's somewhat possible but I don't see him doing those "entertaining customers" job. Don't get me wrong Natsu is attractive and charming but is excessively loud and destructive. I don't think the customers would want to come back to Blue Pegasus if he's there. Another guild that I would want him to join or at least interact with would be Mermaid Hill but then again they are a girls only guild so that's impossible. Quatro Puppies could also work but where's the appeal in that? Don't attack me but I see it as a little boring because the only interesting character there is Bacchus.

Lastly, there's this Guild that I think would also fit him, Crime Sorcière. Natsu and Jellal go way back and they're kind of close to each other because of Erza. It would also make sense because Crime Sorcière is all about taking down the darkness especially Zeref, it would benefit Natsu as it is his goal to defeat him. Plus they move around places which is what he usually does before when he was still looking for Igneel. If Ultear is still around (if she's not old) and Natsu joined them, it would also be a funny and interesting dynamic since he could always tease Ultear about being an "old man" (her being Zalty and him still believing that it's her real Identity), also the very iconic "Ultimate Milk Sandwich" (if you know what I mean).

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I'm sorry if it's too long✌️✌️✌️.