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I'm of the opinion that until Guzma is revealed to be making an appearance, it's not even worth speculating on who is going to voice him. That said, the names that have been mentioned are solid picks. Real shame is how long they waited to even reveal him, much less Plumeria's 15 seconds of fame back in Episode 104.
I feel its not too much out of the norm now for such a late arrival. N wasnt until after the league and Lysandre wasnt until right before the league. So Guzma is likely coming in at right about the same time.
As for voices I'm not well versed in japanese voice actors enough to realy have an idea although i do know daisuke ono's and i feel hes a solid choice.