User blog comment:FairyRaven/Fairy Tail Chapter 440: God Serena, Review and Commentary/@comment-5303512-20150615081428

I kinda like the chapter too, if Serena is indeed Yuri in disguise or something and he truly turned over a new leaf, then we are hella screwed with him becoming part of a team that has 11 more guys that are as strong as Ishgal Kings are.

Well, the chapter say that These 11 Shield guys are equivalent to that of one the 4 Ishgal Kings Serena has defected so this implies they are stronger than the other 6 Wizard Saints.

Besides these are the protagonists I can see being remotely challenging for the 12 Shields, (The other 3 Ishgal Kings, Gildarts, Laxus, Jellal, Natsu, Erza.) Possibly (Gajeel, Gray, Mira, Jura, Mira, the last 6 wizard saints).

Also there is still this Emperor Sprigan of Alvarez that is mentioned, so maybe he is more OP than these 11 shields plus one Ishgal King?