I'm definitely doing pre-heresy Ahriman as the scarab occult leader using the chaos terminator lord kit (love that cloak). I will probably do a magnus but he may be as a display only model depending on the options in the grey knight codex when it comes out. (If there's a rock-hard mephiston style character, I might use him as magnus).
The others I'm less sure about as it depends on what I want/can have in the army. The Pavoni captain (Hathor Maat I think, I don't have the book with me?) would make a good apothecary if that's an option etc. I'd like to do a terminator command squad with all the named characters from the different cults.
Any storm troopers will either be modelled as prospero spireguard or as pyrae automatae.
I'm going to model a few marines as having trophies from dead Space Wolves to reflect the battle for prospero.