Ooh, interesting topic. Well, to throw in my ideas here:
Vincent Martella as Kota Kazuraba (Gaim)
I'm not familiar with Phineas and Ferb, but I do know this VA as the voice of Hope Estheim in the Final Fantasy XIII series and he is a great voice actor with a lot of youth to his voice.
Jason Marsden as Kaito Kumon (Baron)
Another Final Fantasy XIII alumni, this one being the voice of Noel Kreiss. He has also done several other roles including the villainous Chase Young from Xiaolin Showdown. I think he could pull off Kaito quite nicely.
Aaron Spann as Mitsuzane Kureshima (Ryugen/Zangetsu Shin)
This one is the voice of Onion Knight in Dissidia: Final Fantasy and Joshua in Kingdom Hearts 3D: Dream Drop Distance. He has a rather high-pitched voice for a male, but it works for Mitsuzane's character and I'd love to hear Aaron Spann as a villain.
Michael Daingerfield as Takatora Kureshima (Zangetsu/Zangetsu Shin)
I fell in love with this man's voice as soon as I heard him play Sesshomaru in InuYasha: The Final Act. He'd be perfect for Takatora.
Vic Mignogna as Hideyasu Jonouchi (Gridon)
We all know him as Edward Elric in Fullmetal Alchemist, among other roles. He'd fit the role of comic relief Kamen Rider pretty well.
Yuri Lowenthal as Ryoji Hase (Kurokage)
Yuri is the voice of Sasuke in Naruto. Can't think of a better person to handle a young man desperate for power and doomed to a cruel fate.
Daniel Fredrick as Oren Pierre Alfonzo (Bravo)
Anyone seen Grell in Black Butler? That guy is completely insane and he has the perfect voice actor to match.
Jim Cummings as Sid (Sigurd)
Jim goes without saying. He's got a whole repertoire of voices and he'd pull off the deceptive and secretive Sid well.
Dave Wittenberg as Ryoma Sengoku (Duke)
I know him most prominently as Kakashi in Naruto. He has that calm, charming voice that Ryoma Sengoku needs.
Tara Strong as Yoko Minato (Marika)
If you've played Spider-Man: Shattered Dimensions, you'd know this incredibly talented voice actress pulled off a superb Doctor Octopus 2099. She'd handle Yoko very well.
Troy Baker as Zack (Knuckle)
Zack is Mister Knuckleman. So is Snow from Final Fantasy XIII. The voice actor would definitely fit.
Matthew Charles as Peco (Kurokage Shin)
Matt voices Jaden Yuki from Yu-Gi-Oh! GX. I can definitely picture him as Peco.
Laura Bailey as Mai Takatsukasa
I first heard her as Tohru in Fruits Basket and later as Serah in Final Fantasy XIII. She has that innocence about her voice that Mai's character needs.
Richard Epcar as Overlord Deemushu
I love him as Ansem, Seeker of Darkness in the Kingdom Hearts franchise. He sounds like the type who could voice a battle-craving Inves Overlord.
Crispin Freeman as Overlord Redyue
Crispin has a cold and calculating voice that would greatly benefit this Overlord.
Paul St. Peter as Overlord Rosyuo
The master of deep voices, himself, Paul St. Peter has voiced characters such as Kurama in Naruto, Cherubimon in Digimon Frontier and Xemnas in the Kingdom Hearts series. He'd be perfect for this role.