Would they just instantly die, or would be driven insane? Also, is it possible that they could survive but be transformed into some kind of Eldritch horror?

I am curious about this because I would like to incorporate some of the lore surrounding the Illithid race (mind flayers) into my worlds lore and alter what is currently known about them slightly.  Essentially; I want to say, that originally, the Illithid race was actually a much more normal humanoid race - not unlike Dark Elves, who sought to ascend to a higher plane of existence. They became extremely powerful in their own right, acquiring vast amounts of knowledge through bonds or pacts with Great Old Ones.  However; at some point, they became greedy. Having vast amounts of knowledge, but little wisdom - they thought themselves to have outgrown their patrons/gods and decided to just take what they perceived to be theirs. After all, they already knew so much, so what harm could it possibly do to know the rest.

This would be their crowning glory, an achievement shared by their entire race. They would pass beyond the limits of this mortal existence and become immortal Gods. With all their knowledge and all their power - they reached out and touched the mind of a Great Old One and in that instant, they knew their mistake. The death toll was catastrophic, billions of them died in an instant from information overload - their minds unable to cope with or comprehend the sheer amount of information that was pouring into them. Those that survived were broken and changed. Their minds linked together in a hive consciousness, their bodies twisted, corrupted, morphed into monstrous forms.

Eventually, they would recover slightly. Enough to start again and build the Illithid empire but they have never regained what was lost ..... for their arrogance and their greed, forever are they destined to rise and fall - and so; in a way they have succeeded, they have attained a sort of immortality, but they will never again be what they once they were.

I am just curious if this kind of thing makes sense and is sort of consistent with what would happen to a person or people who dared to touch the mind of a GOO.

Thanks

It certainly holds up as a homebrew lore.

It is surprisingly similar to the story of the raven queen and shadarkai.

At the end of the day as the DM it is your port of call, you could say they need to make a save i a player did it and if they survived they gained a boon but if not bad things happen. 

A thing to consider is that the ideas of Old Ones are based of the Cuthulu mythos. With them you would never die but even learning they exist drove the individual insane. Looking into their minds would do that 100x mainly because the way they exist let alone think is entirely alien to the human mind and our minds break trying to piece it together in a form the brain could understand.

Now saying that these enteties were in the mythos of great power and often could warp creatures or spawn new ones, so if someone looking into their mind, after going insane, they could if the god thought the ant on the ground that is the human was worth any attention maybe would alter their form. But as a warning the idea of the cuthulu mythos is humanity does not matter and is powerless in the greater universe full of entities who we are but ants to and our want and desires and feeling matter not to the uncaring void. So not great for the whole heroes saving the day ideals that D&D can bring if you went that route with old ones. 

On that note how I have DMed them in the past is the player will gain a number of indefinite madnesses the number of times based of i think it was 5 high dc wisdom saving throws. To reflect what they learned they gain the legend lore spell (cannot recall if i set a limit though it did do psychic damage as they access a heavy broken set of thoughts and memories of their time in there) and you could totally warp their body.  

But as I said there is no hard or fast rule for these things like your idea sounds awesome. My only comment would be what seperates these great old ones from gods. As traditionally great old ones are unfathomable eldritch horros who tend to work on very basse levels interacting through cults though they use them more as tools then people they trade with (nyarlathotep being a major exception not to the using as tools part more the they are more akin to eldritch loki than some of the others who are more forces fo nature in attitude) while gods are the more uber powerful entities you treat with and bestow gifts to those they deal with. Again not saying dont do what you are doing but may be a question worth asking. 

But a TLDR answer would be as DM your game your rules if you want a result based off people accessing the mind of an old one go for that result.