Gonna keep tossing out Agent ideas until one of them sticks.
Is Lord Solar Macharius available as an Agent? Alexander the Great in Space seems like he would fit in perfectly to help Nihilus reclaim worlds as fast as possible.
I am not sure where the idea that we can just get random people as Agents from the Emperor is coming from. He is not really involved in the affairs of the material Imperium. When we asked him for Agents, his offerings were his Greater Deva. And we picked Celestine specifically because she was the one with the best ability to influence the Imperium! If we wanted someone who was specifically a really good general,
Rogal Dorn was right there.
Also like keep in mind that our position distorts how common we see T1/T2 characters as. I don't think the Imperium normally has
any T2 heroes active. There's like the loyalist Primarchs, but none of them answer to the Imperium. The T1 heroes who are active are well-known to the Imperium at large. And because of that, everyone has opinions on them. Like if he hadn't been abducted, we could have Belisarius Cawl who is really good at discovering new technology, digging up relics, adapting xenotech, all stuff we wish the Mechanicus did. But, yanno, they all know who Cawl is and the conservatives hate him. He would be
worse than useless at influencing them, because he's already their opponent and aligning with him would make our position clear.
So maybe we could, on our own, find some super-firebrand preacher against the evils of the Imperium. But this person might not exist, and even if they did they won't be a convenient way to take over the Ecclesiarchy.