I thought Mashima would give Gray a customized version of Invel's ice armor as a buff, but so far that hasn't happened. I feel that armor is too powerful a spell to be forgotten.
Perhaps in this arc Mashima will remember this spell when writing Gray's fight.
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Regarding this arc, I feel that the first part will focus on neutralizing the Alchemists, while the secondary part will be how to deal with Vierness.
Of secondary plot, the zodiac keys must represent something important. Keys would likely be seen as important materials for Alchemy. Remembering that all 12 Zodiac Keys must be present in this arc, which includes Aquarius (and probably Brandish).
Perhaps the Zodiac Keys are used to improve Athena's performance against Vierness.
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I hope that in this arc Mashima gives us the combination of Taurus + Scorpion or Taurus + Cancer. I feel that any combination of Star Dress
with Taurus will turn Lucy into an excellent close-range fighter.
As for Wendy, I'm extremely curious how Mashima will handle her. Continues with the power progress she had with Irene's help or reset her evolution according to her "old" power level (without Irene). Everything will depend on which opponent Wendy will fight and how well she performs.
Hunm, Erza, I think Mashima wants to forget that the Nakagami armor exists. It's a little funny how no of armor she acquires or enchantment surpasses Nakagami in terms of power and utility.
Natsu... I believe he will remain in the same inconstancy of power. One moment he's a god, in another scenario, he is defeated or takes major wounds against average mages.
I think only Sting and Rogue will be relevant to the main storyline of the arc, especially Sting. Mashima seems to like the character.