Just for fun, I thought it would be interesting pick who I think would be the best choice to voice the Spriggan 12, and I will explain why, so are straightforward while others will be further explained:
August: Takayuki Sugō, because given how wise and badass August has proven himself, he deserves to be voiced by someone who has voiced many wise and badass characters, and the only one I can think who hasn't yet voiced for Fairy Tail, would be Takayuki Sugō. Who has done the voices for Old Man Zangestu from Bleach (And technically Yhwach if the series ever returned), Hashirama Senju from Naruto, Shiryuu from One Piece and Jirocho Doromizu from Gintama. All extremely badass characters, some wise, some ruthless all badass.
Ajeel: Ryōhei Kimura, due to Kimura role as Judar from Magi, given their violent and arrogant nature Kimura voice fits Ajeel perfectly.
Bloodman: Takaya Hashi, due to past role as Kakuzu from Naruto, his cold cruel voice would be a good choice for Bloodman.
Brandish: Fumiko Orikasa, (first I like to say it can be challenging picking female seiyu, because I lot of them could fit in, so I picked the first one that came to my head) I think if Fumiko used her Momoi voice from Kuroko No Basuke, while Fumiko has done the voice for Rukia for Bleach and Kyubei from Gintama, I think using the same tone as Momoi would suit Brandish.
Dimaria: Akemi Okamura, as I said at top random choice out of the back of my head, but I think Akemi would be a good choice for Dimaria, as she has done the voice for Nami from One Piece, who is at times, cocky, mocking and has an high opinion of herself, but also can be emotional at times too.
God Serena: Tomokazu Sugita, I think that Tomokazu for God Serena would be a good choice, because Tomokazu has done the voice for Gintoki, and for those who watch Gintama, we know when Gintoki not being the serious badass, he can be a comical jackass, who thinks of a high opinion of himself at times, and can be condescending at times.
Invel: Yoshimasa Hosoya, Hosoya has done the voices for stern, authority like characters, such as Hyuuga from Kuroke No Basuke or Reiner from Attack On Titan, they both give out a stern outlook aren't taken back by most things, sometimes. So he would be a good choice for Invel.
Irene: Miki Itō, Miki is long provided a role for strong, independent and some crazy women, such as Android 18 from Dragonball Miyo Takano from Higurashi, and Ren Gyokuen from Magi, Miki would be a very good choice for Irene.
Jacob: Rikiya Koyama, due to Jacob character, as being a mix, between serious, blunt and comical at times, I think Rikiya is the best choice for him, as he done the voice for Starrk from Bleach, Yamato from Naruto and Kyros from One Piece, all serious, but sometimes comical characters. (Plus, if you look at this picture of Rikiya, his arm looks like its holding a pistol and shooting, like when Jacob does when he is holding his invisible gun https://en.wikipedia.org/wiki/Rikiya_Koyama#/media/File:Rikiya_Koyama.jpg)
Laracde: Hiroshi Kamiya, I was thinking that Kamiya would be a good choice for Larcade, because Kamiya, has always played a role, of serious, somewhat polite characters and dangerous people, such as Levi from Attack On Titan, Law from One Piece, and Akashi from Kuroko No Basuke, and since Laracde is at times, somewhat polite but is also dangerous to others as well (Power that can kill anyone whose had sex, that is the oddest but at the same time, one the most extremely effective powers I have ever heard), he Kamiya would suit him.
Neinhart: Masaya Matsukaze, since Masaya did the voice for Shin Hyuga Shaing in Code Geass, where his character was a real arrogant, twisted individual, who enjoyed seeing chaos and can be very condescending to others, I think he would be a good choice for Neinhart. And I'm aware that Maya does the voice for Ren, but he could voice more than one character.
Wall: Hiroki Takahashi, Wall gives out an arrogant, violent persona at times, so I just keep imagine him being voiced by Hiroki who did the role of Squalo from Katekyo Hitman Reborn. (Plus tell me after watching this video of Squalo being himself, wouldn't you think Hiroki would be a good choice for Wall).
Tell me what you think, and tell me if you think who would be better if you disagree.