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

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

the problem with it is twofold, really:

A) the Emperor can barely keep up the astronomican to the "brightness" needed to cover the current Imperium with the archeotech psychic amplifier in the Throne and a great deal many other psykers to assist him, removing him from it (and thus extinguishing the astronomican) will throw the Imperium into utter chaos

B) you still need a way to get the Emperor into it, and I'd like to see the way you would go about bringing this... thing into the Imperial Palace and before the Golden Throne to transfer Him into it

Though B is more of a diplomatic issue ... if a Horusian Inquisitor would gather sufficient clout and reputation to move ahead, they might be able to secure transfers, lapses in security, sabotaging surveillance, placing allies and deep cover infiltrators in strategic positions, and so on -- all over decades so as to limit drawing suspicion, until they're ready to penetrate the Sanctum with a swift and brutal strike.

Undoubtedly, the Thorians and Amalathians would move against something like this immediately if it were to be discovered, but who can say when the instigator's true motives would be unveiled?

This could actually make an epic adventure for a high-level group of Inquisition PCs.

National Treasure 40k: Inquisitor Niklaus Cayje is going to steal the Emperor

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

I didn't say it would, work, but You know at least some inquistors might be willing to give it a try tough.

Also: Human centipede. Eeew. Imagine the emperor's reaction when he would wake up in a sown together body.:

E: " By the unholy beast of the warp! Why am I ass to mouth with myself?!"

National Treasure 40k: Inquisitor Niklaus Cayje is going to steal the Emperor

E: "Wait. Put me back!. I'm not dead yet! I feel happy!!!"

I was wondering if maybe the Tau have some hidden psychic research center on one of their worlds.

Earth caste scientist: "Those gu'ella can do strange things with their minds. We must investigate this!"

Fire Caste warrior: "We need to study and in time learn to wield this weapon!"

Ethereal: "Oh no we don't!" *

* then again, you never know with the ethereals.

Well, the Tau themselves aren't psychically active -- however ... I could certainly imagine a top-secret research facility where captured psykers (or "volunteers" from Tau-controlled human colonies) are studied for the Greater Good.