How else was Zeref going to revive Natsu? I mean, sure Zeref might have tried another way...but Etherious is probably the most effective way regardless.
We don't know how else he could have done it because this was the way it happened. Heck he might have not even succeeded if he wasn't immortal. Also, you're missing the point. Even if Natsu was an Etherious, Zeref wouldn't have created the others. The reason for their existence was to kill him and Natsu came last. So assuming he created END he wouldn't have created the other demons.
With Mavis there may have been differences, I admit. but Mavis would have still met Yury and the others. Who knows what would have happened. They might have taken longer to train after they lost to that Phantom Lord guy, but that wouldn't have been that much different. Mavis may have never used Law or any of the fairy magics, but even that is negligible.
No, it would have been very different. Mavis and the others wouldn't have been able to defeat Phantom Lord (I know that wasn't the name at that time) without Zeref teaching them magic. They were taking on an actual guild. Mavis even said that they wouldn't be able to defeat the master even with Zeref's teaching so they made a plan to trap him. Also, without law, Mavis wouldn't have won. Without Zeref's teachings, Mavis and the others probably would have never created a guild and even if they did, they wouldn't have been strong enough to be as relevant as they were.
Ankhseram's curse was described that the more you valued life, the more you take away lives of others. That's why its known as the contradiction curse. I find it extremely hard to believe that Mavis would have spent 6 years after the war, and not take a single life during that time. Even if war dulled her sense of valuing of life, she's had 6 years to spend with her friends to remedy that. I mean, I might be wrong. But I have my doubts.
You may have a point but the fact is that, at the moment, it is canon that Mavis was cursed by Ankhseram. This can't be disputed until proven otherwise. Doesn't mean Mavis was cursed in exactly the same way Zeref was. After all, the circumstances under which they were cursed are different. Mavis isn't done telling her story either but we do know that she was sealed in an ice crystal and we can assume she did this herself since she instructed that they put her grave in tenrou.
Acnologia destroyed most dragons during the Dragon King's Festival and he used a special type of dragon slayer magic to do it. No other Dragon Slayer could have stolen souls from dragons like that. Also, none of the other dragons slayers were as strong as Acnologia. I think its safe to say that the stronger dragons can hold their own against the dragon slayers. Acnologia's dragon king festival was called Ragnarok. If you have an event named after yourself referred to as the end of the world...that's seriously got to mean something.
How would the dragons still being around not effect the storyline? The dragons ruled the world. Acnologia alone could rule the world, and Igneel is almost on his level. Just 4 dragons were able to wipe out the entire face system, which was Ishgar's defenses against Alvarez. They did so in a matter of minutes. If there was even 100 or so dragons of that level, humans would most likely be living under the rule of dragon. Half the threats in this series can be resolved if some human tried to get some help from a nice dragon
Again, Acnologia wasn't the only dragon. Sure they might not have been able to steal souls like him but it was also stated that the dragon slayers of old did kill dragons, only that Acnologia killed far more. Not saying Acnologia isn't powerful or anything, but the other Dragon slayers were powerful too.
As for the other part, you're missing my point. Sure things would be drastically different if dragons still existed but how different would they be? And how does that affect the current story? If dragons were alive, the World would be different but Fairy Tail isn't a story about the world, it's focusing on a specific part which is the guild. See in all likelihood, things would have still turned out the same way if dragons existed. There's no guarantee that dragons being alive would have made Fairy Tail as a guild and the other important aspects different. We were told that some dragons were in favor of co-existing with humans and we were told that after they teamed up with humans and taught them DS magic, they began to win the war. So if Acnologia hadn't slain all the dragons, then they would still exist but those would be the ones that are not after killing humans. Do recall that some dragons did in fact survive for a while and those were the ones that took up foster kids. It's not like they took actions that drastically changed the world or anything so dragons being alive doesn't necessarily mean the story would have been entirely different. Sure the standard of power would be different but they probably wouldn't even interfere much.
Without Ankhersam cursing Zeref, then Fairy Tail as we know it would have been something entirely different if not nonexistent.
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I don't have a proiblem with Ankhseram being involved in the plot, like showing up to talk to Zeref at the end of his life. I just don't see Ankhseram being directly involved with his actions. Like I don't see him going around picking fights with Fairy Tail. Why would he? He's literally the god of life and death. I see his role as being more of a divine judge.
Well that's a fair point but the fact is that Mashima has a lot of space to fit in a story for Ankhseram. You might feel this way now but that's because we know next to nothing about him. It's not like Acnologia and Zeref were clearly written out characters from the beginning of the story. We found out about their involvement and how relevant they were as we went along. The same could happen for Ankhseram.