Imagine Shinzaya being that ship where the more dishonest(yet loyal) one wouldn't be Izaya himself, but rather Shinra. Think about it, Shinra hid the truth about Celty's head from her for years. He lied to her so she wouldn't leave him. Actions that were driven by a fear of abandonment.
With Shinra and Izaya their weird relationship has always been straight forward, and Shinra's never had to mask the kind of person he is around Izaya because the only person whose approval to him ever really mattered to him was Celty's.
However, if you turn that devotion onto Izaya, it flips their dynamic entirely and raises a lot of interesting questions about how Shinra developing feelings for Izaya would impact both of them.
Because Izaya isn't like Celty, he's colder and much more volatile. He's is the type of person who pushes people away when they become too close because he's afraid of vulnerability and letting somebody in with the risk that they will betray or, more specifically, abandon him. (They both have abandonment issues in my book, issues which manifest in very different ways.)
In canon, as tragic as it may be, the only reason Izaya allowed himself to get as close to Shinra as he did is because, speaking objectively, Shinra is closed off from the rest of the world, and, in a sense, emotionally unavailable to him. As we saw from the hospital scene and back when it was revealed why Shinra threw himself in front of a knife for Izaya. The whole reason Shinra asked to start a club with him even was because Celty had told him he needed to make friends and wanted him to socialize.
The constant theme in their relationship is that Shinra is the one person Izaya can't get a reaction out of or manipulate.
Meanwhile, Shinra can practically read Izaya like a book. He knows Izaya better than anybody, and, hell, he's probably the only person who could truly hurt Izaya if he wanted to.
With anybody else, Izaya always finds a weakness and exploits it. But Shinra? He can't do that with Shinra.
So this begs the question:
If Shinra fell in love with Izaya, would it make Shinra more vulnerable to being manipulated by Izaya? Would it make him care more what Izaya thinks of him?
I think yes... but no at the same time.
Even in ShinCel, Shinra doesn't let things affect him the way most people would.
It is gathered that he carried on a certain appearance in front of the person he loved. He gladly joked about being happy to be a puppet for her and he also has no qualms becoming a villain if he needs to.
This is why I consider Shinra a true neutral because he doesn't have a moral stance one way or another.
Say, if Celty told him to murder somebody, he would without a second thought. But Celty is a good person, so naturally she influences him to be good.
But what about Izaya? THAT'S a much more complicated matter.
Because as we all know, Izaya is by no means a saint. He laughs at chaos, he orchestrates gang wars, and he encourages people to do both bad things and good things alike, he helps both cops and Yakuza. And if there's a weakness he WILL exploit it and fuck with people's feelings.
And yet, when it comes down to it, the novels explicitly state that Izaya isn't the type of person who would kill somebody without batting an eye. He cherishes the one genuine connection that he has. Shinzaya izaya ironically choosing not to use shinra even if shinra said he were okay with it would be a testament that Shinra IS in fact the exception
The interesting thing is that Izaya appears like a typical villain on the surface, one who doesn't care about anybody. But when you place him beside Shinra and actually analyze Shinra's character you'll know that a lot of Izaya's behaviors in comparison are purely performative.
Shinra has capacity to become a true villain, because unlike, Izaya, Shinra has no issue getting his hands dirty if he finds a reason to.
Izaya, on some level, recognizes that. He himself even joked about the idea of Shinra becoming a psychotic serial killer.
And yet the truth is I don't even think that Izaya himself knows what all Shinra is capable of, or the extent to which Shinra will go for the person he loves.
Now, in terms of Shinra's feelings...
When Shinra truly loves somebody, it's intense, the person completely consumes his very existence. So if you give Shinra feelings for Izaya and it's not unrequited, their relationship has the potential to become extremely codependent from both ends, maybe even slightly toxic at times.
Because, just like Shinra, Izaya doesn't love people in a normal way either. It was canonly theorized that he his love would be very dangerous. And yet compared to Shinra, it's much more normal, which says a LOT.
In terms of Shinra's love for Izaya itself, the intensity of it probably does not change.
Before, Shinra was honest with Izaya about his nature. He had no reason to lie to Izaya about anything because, back then, his feelings were for Celty, but now they're for Izaya.
So when Shinra first realizes his feelings, there's no world in which Shinra would just casually admit this. Ever.
He could with Celty because Celty was Celty and they had an established relationship.
Izaya would immediately push Shinra away and put up a wall between them, not even because he hates Shinra, but because vulnerability terrifies him.
So for their relationship to remain stable, Shinra decides that letting Izaya believe what he wants is the only logical course of action to him.
In fact, he will probably even humor Izaya's conclusions, just because it's safer to Izaya that way(no, it's actually just safer to Shinra because it means he has an excuse to stay.).
As a doctor, however, Shinra finds any excuse under the sun to treat Izaya's injuries, or, more specifically, to just be around Izaya.
But here's where it gets a bit dark.
Shinra's lies do not just extend to hiding his feelings for Izaya.
They also extend to everything else.
This means he becomes secretive, in a way he's probably never had to before, not even with Celty(and, let me tell you, hiding the information about her head from her was HUGE).
Beneath this doctor persona, there may be a genuine, albeit slightly selfish, desire to be the only one to treat Izaya's injuries also, to the point it extends into obsession. even take Izaya to a hospital and Shinra will probably become restless because he's insistent they're not doing their job correctly.
Because when you look at the bigger picture, Celty is immortal, a supernatural being who can be stabbed by somebody and heal from it immediately(not counting the matter with her head)
So, over all, Shinra never had to worry about her dying.
Izaya on the other hand is just a human, he can bleed and die like anybody else. And it's not just that, but his line of work is dangerous.
Izaya doesn't just patch people up like Shinra or deliver items to people like Celty. No, he actively uses people's dirtiest secrets against them and spreads it around like wild fire.
There are probably plenty of Yakuza that despise him.
Shinra knows this, he knows that he has enemies. In the past, he had no interest in keeping up with the people Izaya was in contact with or who he pissed off, but now, knowing these things would become his top priority, knowing both his enemies and allies alike.
He would pay close attention to everything, and he might even end up going through his files when he's not around, going through his computer and his emails. Namie, of course, wouldn't care, she would let Shinra do these things.
Izaya, on the other, has no clue about it. In fact, he may be too confident to think he already has Shinra figured out and that there's nothing to worry about.
All the while, when Shinra's not stitching up patients, he's actively going behind Izaya's back and doing things on his own.
This goes beyond just reading his files too, and could also eventually delve into a form of "damage control" and taking matters into his own hands behind the scenes, which can be both a good and bad thing depending on each situation. It could even extend to murder if he felt it was necessary.
Izaya of course, is not an idiot, he would catch on eventually.
And depending on the scenario of which Izaya makes the discovery, there's a chance Izaya could lash out for once, especially if he was concerned about it putting Shinra's life in jeopardy.
Shinra is the one person he cares about, the one exception to the way he messes with people. We as the readers know Izaya, despite everything, he would never forgive himself if Shinra got hurt because of him. We saw how he reacted when Shinra got stabbed by Nakura. That event messed him up for life.
While there is always the chance Izaya might use Shinra in a situation out of curiosity, watch how far Shinra would go for him, my instinct says he would do the opposite, telling him he didn't ask him to do any of that and for him to stay out of it out of a surprisingly genuine desire to protect him.
Shinra on the other hand might begin to reason to himself that Izaya's love for humanity, and by extension his job as well, is an obstacle for him. It's an obstacle to what he wants with Izaya and
Doing "damage control" and going behind his back to act on his own, is likely something he's rationalized as another way of connecting to him.
Granted, if Shinra had his way, Izaya would probably never look at another human again and he would only focus on Shinra. But we know that's not going to happen because he's Izaya.
So Shinra takes what he can get. Actually, Izaya freaking out over Shinra's involvement would probably validate that desire since Izaya rarely ever gets angry. Shinra knows he's special to Izaya even if Izaya himself won't admit it.
I do think that if Shinra's feelings are mutual and Izaya does in fact love Shinra back, it would take a LOT for Izaya to ever confess that.
All the time that Shinra was focused on Celty, Izaya was envious of their relationship, just like he was jealous of Shinra's friendship with Shizuo. He got hurt when Shinra hung up on him in the hospital and brought it back up three separate times.
Down deep Izaya WANTS to be special to Shinra.
But if Shinra actually turned that focus onto Izaya, Izaya would run away and maybe even ghost him because he's afraid of true connection. Because being truly loved and accepted for who he is isn't something Izaya is used to.
Even then, Shinra would still find ways to insert himself into Izaya's life.
The fact is Shinra recognizes Izaya's fear, maybe even more than Izaya himself does.
He knows that, for once, Izaya isn't just saying harsh things to be cruel but because he refuses to be vulnerable with somebody and risk his fragile heart being broken.
It would actually take something extreme for Izaya to even remotely admit to himself that he's in love with Shinra.
Something like an incident where Shinra gets hurt, where his life is weaponized against Izaya.
You might have an arc similar to what we saw with Masaomi and Saki and how much time Masaomi spent avoiding her after she was injured.
Izaya's speech about how "your past becomes your god" makes me often think he was actually subconsciously referring to Shinra in reference to Nakura stabbing Shinra.
It would be like that but ten times worse. Izaya visiting Shinra the same way Masaomi visited Saki, while simultaneously burying himself further into his work than ever before, using his obsession with humanity as a scapegoat to avoid the one human that truly matters to him.
Talking about humanity in front of Shinra even just to spite him, as if he's trying to make a point that Shinra means nothing to him.
But the worst thing is Shinra doesn't even mind it because at the end of the day, he knows that Izaya will come back to him eventually.