In my opinion, here's my choice on either voice actors or types of voices.
Eustass Kid- Mark Hamill(just a suggestion though) either that or he has to have a insane voice. I mean he's crazy for pete's sake!
Trafalgar Law- Michael Rosenbaum
X-drake- I think he needs an almost authoritative, serious voice. You don't really see him laughing in the manga or anime anyway.
Apoo-I'm not sure, but I think he might need a sort of weird or unique voice.
Bonney- a high pitched(but not shrieky voice). Maybe even a sort-of childish voice.
Basil Hawkins- I think he needs a calm toned or unemotional voice. Maybe even Carl Lumbry as his voice.
Urouge-a deep voice most likely
We already know Luffy and Zoro's voice actor/actoress. I'm not sure about Killer nor Bege.
Got any suggestions for the voice actors for the 9 supernovas?