Disclaimer: This is my personal opinion. To the extend of my knowledge, GW never officially stated any way to revive the Emperor. Until they do (and with that basically bring back the Emperor), it can only be an opinion.
It has not been that anything has been discussed between Guilliman and the Emperer, I personally have my doubts that any conversation took place. The reason for that being that the Eldar tried to talk to him psychically, with no success.
From the book Throneworld:
They were so close to their goal. Lhaerial could feel the lessening of the Palace’s psychic defences. They were nearing the centre of the wards. She called out with her mind to the Lord of Man. There came no reply, but lines of Adeptus Custodes running at them. The great gates were far away. For a moment her heart faltered. She could not succeed. But she must try.
Copied from this reddit article
Could be that the Emperor did not want to answer but the question arises why he did not communicate during 10000 years to any of his most loyal servants he likes most (Custodes) and then suddenly talks to Guilliman.
As for your question if he can be healed, look at different ways:
- Current human technology
- Other sources of human technology
- Xenos technology
Current human technology
With the current human technology, it is probably not possible to bring back the Emperor. Even worse, they currently have problems keeping the Golden Throne running.
By late M36, the Golden Throne began to require more and more sacrifice of Psykers to remain functional. By early M41, four times the amount of Psykers were required to maintain optimal power levels of the Throne. In the last year of M41, Techpriests discovered failures in the mechanisms of the Golden Throne that are far beyond their ability to repair.
From the Lexicanum article on the Golden Throne
So, humans are currently not able to revive the Emperor. He'd probably not be sitting on the Golden Throne otherwise (duh).
So, let's look at
Other sources of human technology
The first coming to mind is an STC. Given that STCs are artificial intelligences with the whole mankind's knowledge at their peak, it could be argued that if human technology were to be able to save the Emperor, it should be contained within an STC. To the question if a full STC is still available, I'd argue yes.
It is AFAIK rumoured that the Speranza has a, dormant, AI and not only an advanced Machine Spirit. But more importantly, I think the Emperor has (and used) an STC. The reason for this is from the book Deliverance Lost. Corvus Corax walks into a room with hundreds if not thousands of ancient relics. It is, in my opinion, logical to assume that if the Emperor could salvage parts from the first warp capable space ship, he'd have an STC.
Given the name of the place, Corax had expected to see lines of battle honours and banners, displays of armour and weapons lining the walls. Instead there were many glass cabinets varying from those small enough to fit in Corax’s palm to some the size of battle tanks, arranged in rows across the hall, each containing an object from across the galaxy and dating back centuries, millennia, tens of millennia. Stepping up the nearest cabinet, Corax stooped to examine the contents. He felt a tingle of static and heard the faint buzz of a stasis field generator. Enclosed within was a small circuit board, its function unknown. On the stand below, a small steel plate etched with plain text revealed its importance: Navigational Circuit from the first warp-capable starship Corax stepped back in surprise. Intrigued, he turned around and found himself looking at the skeletal form of a wheeled vehicle, barely large enough for a normal man to sit inside. Its balloon tyres made up the greater part of its bulk. Corax stepped up to examine the title plate. Titan Rover The primarch was not sure what to make of it. It certainly looked like no Titan ever produced by the Mechanicum, which were towering war machines tens of metres high. He looked more closely at the vehicle, but could not see anything that might be a weapon mount. With a grunt of confusion, he moved on, eyes passing over various technological artefacts and coming to rest on a glass tube filled with a pulsating liquid coloured a deep blue, located about a dozen metres further down the hall. The words beneath, though written in Imperial Gothic, might well have been an alien or lost language, for all the sense Corax could make of them. Mendelian Eukaryotic Genesis Formula Raking his fingers through his hair, which had slipped across his face, Corax straightened, bringing something else into his eyeline. It was a small cabinet, less than half a metre to each face, but its positioning on the central aisle seemed to mark it out as of particular importance. Within was a broken piece of pottery. It was utterly unremarkable, shattered into eight curved shards of crude unpainted clay, marked with fingerprints and dents. Piercing the parts together in his mind, Corax worked out that it was a bowl of some kind. He heard the whisper of the doors opening and turned back to see Malcador entering the hall, striding with purpose. His face was flushed with blood, his eyes bright and alert. ‘What is this place?’Corax asked. ‘What manner of victories are celebrated?’‘The most important kind,’said the Sigillite, joining Corax beside the shattered bowl. He pointed with a skeletal finger at the contents of the cabinet. ‘One of the first pieces of pottery ever made by human hand.
He probably did not have it on Earth and hid it very well, thus it has not been found. But I think it should be around and he used it when he armoured the Space Marines. The reason for this comes from a book where Horus meets other humans and they had an STC (or only part of it) and it created similar armour as the Space Marine have.
As for the Xenos
There are currently two xenos factions that could maybe bring back the Emperor: Eldar and Dark Eldar.
The followers of Ynnead brought back Roboute Guilliman and, should Ynnead want it, could maybe bring back the Emperor. It certainly would be the best chance to revive the Emperor. Not only heal his body but also his soul from millennias of torture on the Golden Throne.
On the other hand, Dark Eldar Haemonculi should have the knowledge to heal the Emperors body. In "Path of the Archon" (if I remember correctly), a Haemonculi revives an Archon after centuries. It should therefore probably be possible for a gifted Haemonculi to heal the Emperor.
I see no other xenos faction that would have the knoweldge or psychic power to revive the Emperor (probably the Old Ones, but they're extinct).
