I think that jumping headfirst into the Horus Heresy would be a big gamble. It may work, but if it doesn't, oh boy, it can fail really hard. Just making the primarchs work on-screen would be challenging. Also, Cavill as the God-Emperor would be fine, but it would also be a waste; the God-Emperor is more like a force of nature than a character. He has very, very little screen time.
Wasn't there some talk some time ago about an Eisenhorn series? I think it would be an excellent choice. It's a great story that has both action and drama, the characters are mostly regular humans, so more easily relatable, it showcases all of the more mature themes in the setting, of which there are plenty, and Cavill would make an awesome Inquisitor.
Biting the hand that feeds IT
