Wanted to make this a thread rather than just a post because I think it's worth having a fairly long form discussion about.
What I mean by the title is that I feel like DC's setting up Superman to have an in continuity refreshing and reinventing in a similar manner to Immortal Hulk and House of X. That said, the "Superman" in question seems to be Jon Kent, in my opinion.
Basically the similar ideas behind Immortal Hulk and House of X is taking a very well known multimedia IP and redefining its role within the structure of the greater comic universe. In both cases it takes these characters/teams and turbo charges their reach, capabilities, and status. And on top of that it sets them into moral gray areas in relation to what they're normally know for or compared to the other heroes. Hulk is now more monster than hero, and out to smash the world as we know it. The X-Men now have former villains among their ranks, and early on fans (and heroes) wondered if they they'd turned evil. And in terms of powers, Hulk is now functionally immortal and linked to a cosmic death god, and the X-Men not only have the most powerful characters in Marvel among them but they're nation is among the most powerful in the world.
So, I'm looking at what we've heard from PKJ in interviews, and read in the books so far, and I'm seeing very similar ideas at play here. The big one is that in basically every interview PKJ makes sure to remind us that Jon's growing power alone is not just going to make him more powerful than Clark or Kara, but that it'll change the power structure of the DCU going forward. And similar to how Hulk and the X-Men are now figures of controversy within their universe, DC seems to want that direction for Jon given PKJ's IF story where it's said that Jon becoming Superman spells doom for Earth because he'll be a tyrant of some sort (implying he has the power and means to do so).
Then we also have the solicitation for issue 1031 of Action Comics where "Atlantean scientists study the wreckage of the Warworld vessel…and make a shocking discovery that could change the balance of power on Earth." The wording is very similar to how PKJ describes Jon's growing power. So this may be either them discovering evidence Jon's staggering powers, or something that Jon may use in addition to his great powers to change the power structure of the DCU.
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So lets answer it. Lets answer "why don't they just use Clark rather than Jon?"
The sort answer is that because he's Clark, and it's very clear that DC no longer wants to do morally ambiguous Clark because they've been burned too many times. From the way PKJ talks about his take on Clark being that perfect aspirational ideal to Superman & Lois being so full force on unambigously good Clark, I think the classic take one out. The one-two punch of Man of Steel and the New 52 getting backlash the way they did seems to have seen DC go full on classic ideal for the foreseeable future with the Clark character. But that doesn't mean that's what they want for the Superman IP in general. Since Jon doesn't carry the same baggage Clark does as an IP he's allowed to take this direction without too many people going nuts. Like, could you imagine what people on here would've said had that been Clark they were talking about in PKJ's IF story? About how Clark shouldn't be Superman anymore because he'll become a tyrant? That wouldn't have gone over well at all. But you say that about Jon and you frame it the way it was framed, and outside of the people who are still mad over the kid stuff the reaction is most just "oh, that's interesting".
And I think the overwhelming success of Immortal Hulk and X-Men plays a big role in them going for this. Like Superman, both are IPs that everyone knows, but have not been as popular as they could and should be for a while now.
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In terms of Jon as a character, the idea seems to be that his heart is in the right place, and he is still that fun and joyous person. However, due to the fact that he's going to be too powerful to *not* do the job, and his dad will be somehow out of play on Earth, he's gotta step up, and the pressure will be great. But apparently he's also a person who doesn't act and react the same way his dad would, so he may do things in such a way that, coupled with his ridiculous powers, may scare the rest of the hero community.
On top of that, there's also the possibility of him using his "insider knowledge" of the future to be proactive in ways his dad couldn't be (and maybe wouldn't be even if given the option), and, again, that may scare people.
On top of that Jon is dealing with some issues from bouncing around time and space for basically his whole life making even his technical home somewhat alien to him. There could be very real debate among the heroes regarding if it's safe for the world because Jon has that much power
You're essentially looking at an 18 year old god knowledge of the future and maybe even new crazy tech from the downed Warworld ship.