Irene, Erza, and Wendy would make sense since this subplot is about Erza/Irene. Also Wendy is involved in the fight as well.
Irene appearing on the chapter cover in her dragon form sounds badass. Also I can see Erza and Wendy on both sides of Irene.
I thought of that too.......
But I thought with Chelia and Wendy on the two sides of dimaria on her cover of vol 56. That sounds too similar.
On the other hand if Erza is standing ready to fight we be too much like the Erza in vol 54.
While having Irene big with all these smaller characters scattered around her - Larcade, Sting, Kagura, Wendy, and Erza.... that would be too much like vol 57...... and having the Erza vs Irene big with all the other characters small or adding Wendy to background would be like vol 55.
Plus if we have a Irene vs Wendy and Erza face off. We just got that with the Gray vs Natsu cover.
A dragon would change things up.
My vote would be something like the below pic with Irene and Erza. Wendy can be in background.
Or like...
^This with a red theme and Erza with her dragon mother. And Erza holding her sword in her Flame pants.