Or create a psyker so powerfull it can establish a mindlink with the Emperor allowing the big E to walk the galaxy again, clad in the form of that psyker (the only one capable of holding the emperors mind)
If it was that easy, it'd have been done before! Imagine this... It's not one super-psyker, it's a single amalgalm body (think Franken Fan + human centipede) that any Magos Biologis and Adeptis Astra Telepathica savant would consider a great heresy of fused bodies and minds. What about the horrible effects that the psykers would undoubtedly suffer as their (still) inferior form struggles to contain the might of the Emperor?
And when it dies, don't let the players know whether or not the Emperor has just been killed again