In Superman #149 (Sept 1961), a memorable “imaginary tale” was published: “The Death of Superman”. It starts out kookily enough: Luthor suddenly seems to repent, synthesizing a serum to cure cancer from a meteorite he finds in a prison rockpile.
Despite this, the justice system is skeptical of his reformation, and rightfully so, until Superman makes a plea during his parole hearing and Luthor is freed from prison. Oh you silly trusting Kryptonian.
Superman even gifts Luthor some swag: a signal watch to for emergency contact. Poor Jimmy Olsen ain’t so special anymore.
Criminals decide to target Luthor as a traitor, threatening his life, so Superman builds him a secluded laboratory satellite orbiting the Earth so he can continue his scientific research in safety. One day Luthor sends an emergency signal to bring Superman to the laboratory satellite and then waylays him with green kryptonite rays:
We then get the typical villain exposition of how he lulled everyone into a false sense of security, was never reformed, and then goes to extra pains to ensure Superman is really dead (including checking for a Superman robot). For that extra sadistic bent, Luthor had even brought Lois, Jimmy, and Perry to the satellite to witness his painful death. An entire page of Superman dying must have been a little traumatic for readers of the time, even if it was an “imaginary tale that may never happen”.
Luthor takes a moment to gloat in triumph and extemporize about his greatness:
Back on Earth, while the Earth mourns the loss of their protector, a nice funeral scene that is also depitcted on the cover with many superheroes and friends paying tribute. Luthor rallies the criminal underworld around him but is quickly foiled by … Superman? Ripping off the disguise it turns out to really be Supergirl, surprising everyone (remember the public did not know of Supergirl’s existence at this point).
There’s a slight inconsistency here, since Luthor is seemingly taken back by Supergirl’s existence, despite learning of it only a couple months before when Lesla-Lar revealed it to him. Anyway, I’ll allow it, since it’s unclear if Luthor will continue to remember this in future Supergirl stories – and also continuity between all DC stories is not very consistent at this point. Overall, this is a nice moment that allows Supergirl to carry on the legacy of Superman in public: the world still has a super-protector, despite the tragedy of losing its champion.
As mentioned before, “Imaginary Tales” are a great example of a DC innovation. Having established increasingly detailed mythos and lore around its characters, they give opportunities to explore what would happen if they broke out of the status quo, without actually harming its established continuity. Parallel worlds, multiverses, and parallel timelines would later become increasingly used for this because they let these explorations live longer than a single one-shot and have richer lore of their own. Of course all this comes at the cost of taking away the “specialness” of the primary world/timeline and stagnating stories with too much “inward looking”.