Souls make up the fabric of the Warp. Most everyone goes there. Particular souls stand out so much (psykers, eldar) they're often devoured. Corrupted souls might get devoured, added to the weft and pull of their deity in the Warp, or might even be elevated into its own distinct entity. There's theories out there about ardent Imperial souls "going to the Emperor" or something but I think that's hogwash.
On the choice of content, if Cavill is savvy, there's a couple things they could do successfully I think:
-Go straight for the Highbrow stuff. Eisenhorn, Gaunt. (Not Ravenor. You don't put Henry Cavill in a box.) Of the two, Gaunt would be the easier one from a "get the audience into it" standpoint, but way more expensive since it's about battles, whereas Eisenhorn is a smaller scope story that's easier to shoot and budget for.
-Go for the easier stuff, but pick the right stuff. Ok so yeah, you end up doing Ultramarines to backend off the success of Space Marine 2. But you pick the cream of the crop like Uriel Ventris, something you can tell a decent narrative that was a few seasons worth of stuff to it, instead of a one off "Space Marine shoot thing."
What they should definitely not do is:
-Anything Horus Heresy. No one who isn't a 40k fan is going to attach to the lore. The Heresy only resonates because it informs the present of 40k.
-Anything current era storyline. Sort of the same problem. Guillman doesn't mean much to people new to 40k.
I think Cavill probably understands the kind of guardrails GWS wants to keep on the IP, so he's probably not? at risk of doing the same thing to 40k that Lauren Hissricht did with the Wither. Like I think he knows enough not to legitimately try to do something like this: