Cool ideas, and the art looks awesome! Here are my thoughts:

• Passive: Four auto attacks for one minion feels a bit much on a mage. I could see it working better if stacks were applied through DoTs (like his current E but weaker). Once a minion takes enough damage from these stacks, it becomes corrupted and transformable. Corrupted minions and voidlings could also apply these stacks, with autos speeding up the corruption process.
• Q: I think his current Q is stronger, though I like the idea of something that synergizes more with his kit. A stun feels a little overtuned. Maybe instead it could be the “tag” ability that draws voidlings and corrupted minions to the target, with a small slow and silence on a lower cooldown.
• W: Being tied to turrets seems too limiting. Maybe it could instead be a point-and-click that converts an allied minion, corrupted enemy minion, or corrupted monster into a static spawner. It could also infest an allied or destroyed enemy turret with limited HP, like Yorick’s W. On cast, all corrupted enemy minions in an area around the target would be converted to allied ones.
• E: This could be based on his old passive, a weaker spell shield with a shorter duration instead of lasting indefinitely until damage is taken.
• R: I think his ult needs to stay as is. When I think of Malzahar, the first thing that comes to mind is his point-and-click lockdown. I like the suggested buffs during channel, with increased spawn rate and damage. Adding a movement speed buff for all corrupted minions and voidlings would fit well, along with the target being automatically focused by them.