Thank you! I appreciate your contributions as well.
At first I didn't get what you were saying about revenge, because E.N.D has literally done nothing ever, to anyone for that matter, but E.N.D created Tartarus, and by killing E.N.D you would avenge everyone who was wronged by Tartarus. I also think that Natsu would lose to Gray because Gray would outsmart him. By losing this way, Natsu would have to get more creative and grow as a character. Character development right there, and Natsu becomes a more formidable opponent, something that would help with his goal and set up the second battle with Avatar. It might flow more like the Oracion Seis Arc did in that everyone gets beat up at first, but after regrouping later on they take down the enemy. I would love it if Gray did a Sub-Zero style ice clone where touching it freezes you, he'd probably comment on how stupid Natsu was. Natsu would most likely melt it, but it'd be cool nonetheless.
You are right about protecting people being powerful reasons for fighting. However, we don't know that Gray isn't fighting to protect others. To protect others from the evil that is E.N.D, to make sure no other evil guild is created by him, to make sure that Fairy Tail is never in any way hurt by Zeref's most powerful creation, the Fire Demon E.N.D again, to protect them from him. It could be some of his reasons. If anything, revenge isn't his only reason. He looked down on Lyon a lot when he became obsessed with killing Deliora(kind of ironic). His father helped him get over Deliora. As a kid he hated Deliora and wanted to kill it, but because of his want for revenge and also his need to protect others from Deliora's destruction in the future, his beloved master died. Lots of parallels I'm figuring out right now. Demon does something crazy that brings destruction and sadness to his family.
Deliora: Kills his family and destroys his entire home, does other crazy things and kills others. He then had to be taken in by another person. This person then dies after stopping him from taking revenge and dying.
E.N.D: Created Tartarus, an evil guild made up of the books of Zeref who destroyed Fairy Tail's home and traumatized a couple of the members, as well as killed lots of people. The guild also tore his family apart and destroyed his home. He then had to go with someone else.
So his past and present aren't too different from each other. I wonder what role Lyon will play with this, because I know he's got to. Which means that Gray is going to be crazy for a little while longer, meaning he or the combination of Avatar will have to be able to take on both Natsu, Gajeel, Lucy, Happy, and that Forcible Arrest Squad. So if for nothing else, Gray will have to at least be on equal terms with Natsu for plot. I just don't see this dark guild who have been built up for this many chapters with us even learning their names to just get blown over in the next chapter. I'm also pretty sure there are plenty of dark guilds that worship Zeref, but Gray felt that this one had the best chance of reaching him, for whatever reason. I doubt he'd buddy up with a bunch of weak sauce dark mages. We also are only seeing the core members, usually a sign of a main group like the 7 Kin of Purgatory, which were all powerful in their own right. Same with 9 Demon Gates, who would stomp the 7 Kin of Purgatory. I kind of hope Avatar is a dark guild that isn't beaten in a single day, the way it has been with all of the others. Mostly because all they have is 3 members of Fairy Tail and not because each member of Avatar severely out classes the former. So the reason Natsu, Gajeel, and Lucy will lose is because they're outnumbered. I totally don't count those grunts with Gajeel as a form of opposition to anyone. Shit, I have totally forgotten Panther Lily. It seems like he can be in human form almost indefinitely in not forever, because he seemed to be chilling out in human form with Gajeel, something he never did before. Lily being there doesn't change anything, but I thought I'd mention the S-class Cat.