I don't think prior roles are the end-all be-all of who will be considered. It's probably more of a guideline where they don't want one of the main actors playing a bunch of other roles like Sabat was forced to do in DBZ, but it's not like it hasn't happened already. Brina Palencia plays both Chopper and young Zoro, and Patrick Seitz played a bunch of side characters already, including Kuroobi. As for the other choices, John Burgmeier is Fullbody who will also be present in the war, Jerry Jewel is Kaku who is only one arc before Brook, and Vic is already so connected to the theme songs, so they might want to differentiate Brook's songs from them.
Whoever they get will likely have to be one of their more established actors, since they need to know he'll be able to stick with it for the long haul and have the acting chops to work with, especially with singing involved. It's highly unlikely they'll have just the guy in mind who hasn't already had some part in the show by that point.
Well, don't forget I also mentioned that if he is Mr. 3, then he would be playing a bigger role in ID and Marineford too. Mr. 3 is not such a minor character when compared to Young Zoro, Fullbody, and Kuroobi (in the grand scheme of the series thus far). Hell, Mr. 3 has almost as much if not more screen time than each of them combined in Little Garden alone…add in Alabasta, ID, and Marineford, and that's a whole lot of screen time (and direct interaction with Luffy). In no way am I trying to say that John B. is the be-all-end-all Brook with my logic, but for me, I think it would be a good choice, and he'll bring a sophisticated, yet playful voice to Brook (Shigure from FB), while also being able to pull off the badass and sad moments (Kurama from YYH). If anything, finding out who ends up playing Brook is one of the things I'm most looking forward to once SW comes out! ^^
Me. I should play Brook. Just give me enough time to finish college and get down to audition
I mean, come on. I'm majoring in Musical Theatre, I'm clearly qualified.
Hehe, I'd like to voice act for Funimation (or any other anime company) too. I dabble in voice work. XD Plus, if I could get any role in OP, I'd gladly do it for free! Having my name and voice in OP would be reward enough…though that's a pipe dream. XP