Flynn Scifo from Tales of Vesperia and Mr. Just As Planned from Death Note. I'm pretty sure you're familiar with those two...
There's also Setsuna from Gundam 00, Ling Yao from FMA 2009, and Death the Kid from Soul Eater (the latter two sound awesome when they get serious).
In other news, I wouldn't mind Rica Matsumoto voicing Red as long as it isn't too similar to Satoshi's after the early parts. If I didn't mind a hint of Satoshi's voice coming out of Ryou Bakura, I probably wouldn't mind hearing a little bit of it coming out of Red.
Romi Paaku should voice Gold... for the win. Hikaru Midorikawa as Silver = AWESOME.
Ack, I forgotten that Light, Death the Kid, and Setsuna were voiced by Mamo. Goes to show I don't know what I'm talking about. xD Sadly, I haven't heard Flynn that much to gauge how awesome Mamo makes him. Remembering Death the Kid, I think Mamo would also be good for Gold. Midorikawa for Silver? I have to hear it. I keep hearing Silver with a cute boyish voice...
Or Rica Mastumoto can do Red with her Ryou Bakura voice. xD I can't help but think of Ken Ichijouji and now Regene Regetta (thanks for that!) when I see Romi Paaku.
Seiyuu Alternatives List:
Red = Miyu Irino (Because... Red reminds me of Syaoran? Yeah, that sounds good.)
Green = Takahiro Sakurai or Takehito Koyasu (Because I love both; there's no other reason. xD)
Blue = Romi Paaku (Why? She did awesome Regene and it can work!) or Rei Sakuma (who does Shampoo and can Blue's personality) or Megumi Hayashibara. *shrugs*
Yellow= Asami Sanada or Ai Orikasa (Orikasa does male roles so she might be able to a genderless (femaleish) voice.)
Gold = Mamo (I can see Mamo really bringing Gold to life with Gold's personality working well with his voice. I keep seeing Gold as Tamaki...)
Silver = Kōki Uchiyama (Best known Roxas in KH and Neku from the TWEWY, but seem to do other
rapeable young boys. The only one I can think for Silver that would make him sound like a guy, but not MANLY.)
Crystal = Rie Kugimiya (I've heard that she's tired of the tsundere roles, so I would love to see what she can do with a serious girl like Crystal.)
Ruby = Hiroshi Kamiya (Does Zetsubou-sensei and Natsume from Natsume Yuujinchou, so I think he would do a good job with Ruby's unstable characters.)
Sapphire = Fumiko Orikasa (Does some tomboyish girls such as Ruki from Digimon Tamers, Sherry from Gash Bell, and Elizabeth from KKM.)
Emerald = Yumiko Kobayshi or Masami Kikuchi (Kobayshi has a childish voice like Azuma from Yakitate!! Japan and Kikuchi does Kanchome and Tenchi).
I didn't do Pearl, Diamond, and Platinum because I don't know their characters well enough to get a sense of voice for them. And this was very educational... And I'll do the rest later, this killed me.
There's also
Keiji Fujiwara,
Tomokazu Seki,
Marina Inoue,
Shinichiro Miki, and
Ryotaro Okiayu that are super awesomely wonderful options. But, we're forgetting one important character... Who will be done by God (aka Wakamoto)?! I say Saki/Sird.