Rewatch the Spriggan arc in anime, I think Mashima did lot of mistakes in that arc, one of the worst been his writing for Zeref,Irene and August(endphase).
Larcade should never been placed alongside with Irene and August, placed to the other spriggans.
Ajeel,Jakob and Neinhart should never comeback either, defeated once was enough, them coming back only damaged their portrayal.
August plot was good, him stomping Jellal and OS was a great feat, later even defeating Gildarts and Cana.
In sametime I would made Irene defeat Erza and co(of course giving Erza great moments but still make her lose against base Irene). Yet I would use the mother plot her too, but let Irene just left Erza defeated without killing her, make her and August not suicide killing themselve.
Rather I would use that time for Acnologia, make August and Irene face Acnologia(Base), let them stall Acnologia and give them proper feats, then I would let Gray and Natsu face Zeref, give Zeref fairy heart for strong magic power, defeat Gray(he give his magic power to Natsu before going down), I would Acnologia go into dragon form and killing of August and Irene.
Final battle would be Dragon Acnologia vs Zeref(give him a final form, as someone who had a book of demons, I would give him a demon form too with fairy heart) vs Half Dragon Natsu with gray power.
Make Zeref sacrifice himself and badly injuries Acnologia and share his power to Natsu:
Natsu kills Acnologia with Zeref power...
That would be the best way to end Fairy tail, giving Zeref and Acnologia great portayal and also good ends for August and Zeref, no lackluster ending, no bad portrayal for Zeref.
It is kinda sad how the arc ended, so great potential got wasted...