The fight that should have happened but didn't, so lets do it ourselves

[Zackra1799]

Lets set the stage:

Natsu and Zeref are currenly locked in a heated final battle, when suddenly Zeref breaks away completly frozen in fear. Lucy and the other look towards the Black Wizard in confusion, Natsu however lunges towards him and with a flaming fist, sends Zeref crashing into the back wall. Zeref mutters only two words as he gets back to his feet, "He's here." Lucy reply's back "Who's here?" only for the sudden rumbling of the Guild Hall to cause even further confusion. Natsu begins to sniff the air and suddenly tells Lucy and the others to get out of here right now. Lucy starts to question Natsu why when suddenly a dark figure smashes through the Guild Hall's ceiling and lands in the middle of the main hall. This figure is non other than Acnologia as he stares Natsu directly in the eye declaring him his next target. Natsu now stands ready to face the Black Dragon alongside his brother Zeref in the ultimate battle to determine the fate of the world.

 

[Ionliosite]

I assume this isn't the Natsu that already one-shotted Acno.

 

[Zackra1799]

Yes, basically this takes place when Acno shows up to Erza and Wendy but instead he heads directly to Natsu and Zeref given they have the strongest Magic powers at the time.

 

[DemonGodMitchAubin]

This is actually a very interesting premise and start to a situation, I'll try to come up with a scenario that makes sense

Zeref immediately realizes he has no time left, he will quickly try to escape while avoiding Acnologia, Natsu also realizes how outclassed he is in the situation and would likely use Dragon Force right away to stand up to Acnologia, Acnologia likely ignores Zeref because he'd want to get rid of Natsu first since he's a Dragon Slayer, so Natsu and Acnologia start fighting while Zeref escapes, Natsu puts up a valiant fight against Human Acnologia, but his attacks aren't working, there is a very likely chance that Natsu uses Savage Dragon Fire Form and begins to overpower the Human Acnologia, forcing him into his Dragon Form, if that's the case, Natsu now is basically outclassed and outsized as he desperately struggles to fight back, he'll take some hits, but he alone can't win, meanwhile during all this Zeref finds Mavis, steals Fairy Heart and is now strong as well

At this point, there are multiple scenarios, Acnologia still fights Natsu, while Zeref ignores them and uses Neo Eclipse, which means his plan succeeds and Time Resets, but there's a more interesting outcome where Acnologia forces Zeref into a fight after this increase in power, in this scenario it's a 3 way brawl which can go anyway honestly, I could see Natsu and Zeref maybe teaming up, and if they did, I'm confident they could win, maybe Zeref shares Fairy Heart with Natsu, could you imagine a Fairy Heart Amped Natsu, that'd be insane, if they work together, they pull out the win, then fight, Natsu wins, if they don't decide to team up, I see Acnologia winning this hands down, there's a legit scenario for each of them to win, but it's very unsure, I don't know how Zeref's Hax would work on Acnologia

 

[1997KD]

its honestly depends how zeref take this situation, he can either try to run away or give uo his plan of rot and give fh to natsu

 

[CloverDragon03]

That sounds like something straight out of a fanfic

 

[The_King_of_Prudence]

He'd do the reset, because that stops Acnologia.