For me, I would for the most part leave everyone who appeared and spoke in the movies alone... except for maybe Lewa. I'm still working on who I would have to get him. Maybe Sam Vincent? There's been a lot of thoughts in my head on how the characters would sound in my head. I'll start off with one post on the Makuta.
For Icarax, I'd probably like to have whoever voiced Shockblast from Transformers Energon do the job. Icarax reminds me a bit of Shockblast in the sense they're both very cruel, evil, and dangerous. In fact, Vican's thoughts of Icarax throughout BL#9 shouted "Shockblast" to me.
For Antroz, I'd probably have Brian Drummond, but closer to how he voiced Transformers Armada Blurr. Even though Blurr isn't a villain, he and Antroz each have their own code of honor. Plus, I personally think he'd sound all right.
For Vamprah, I'd like to have... I'm just joking.
For Mutran, I'd probably go with Colin Murdock, voice of Transformers Armada Thrust and Energon Scorponok... but probably closer to how Thrust sounded. I think it would just fit him all right.
For Chirox, maybe Paul Dobson. He's done a variety of voicework on various roles, I think he could pull off something sinister sounding and maybe add a shrieky element like that of Nidhiki... but no so much high-pitched.
For Krika, this guy has so much potential unrealized in the actual story. He holds a lot of secrets and regrets, and his tragic story has a tragic end. For whatever reason, I could imagine him voiced by possibly David Kaye. That's just me.
For Bitil, I'm kind of stumped. Probably Terry Klassen, the voice of Transformers Energon Six Shot. Or possibly Brian Dobson.
For Gorast... the choice for her is kind of difficult for me to say. Maybe Gina Torres, who is the voice of Airachnid from Transformers Prime. Originally, I imagined Gorast to be somewhat like Roodaka (so in turn, Kathleen Barr), although Roodaka and Gali share the same voice actress, so... I wasn't sure. Then years later, hearing the voice of Airachnid, I thought I could imagine Gorast sounding like that. Gorast and Airachnid, like Icarax, are both ruthless and dangerous. Plus, they're both kind of ambitious and bordering insane, although they have different motives. For one thing, Gorast didn't plan to overthrow Teridax or anything. Icarax did, but this isn't about him.
And that's about it for now. If I think of my options for other characters, I may return.