I haven't really posted on any of my collections of ideas in quite a while, because I've kind of fell out of touch with writing things recently. This idea is about if Subaru from Re: Zero was Flugel, and I'll be going into detail on the characters I want to accompany Subaru later on, how his personality should change, even if only slightly, to suit him being the sage, and how the story should generally go.
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To start things off, you have to decide at what time Subaru gains the memories, experiences, and authorities that Flugel presumably had. I'm not sure which arc would work best for him gaining Flugel's memories, so I'll instead give you a brief idea of what it would look like for some arcs if he were to gain them then.
I'd personally prefer if you went with arc 1 or arc 3 for Subaru gaining Flugel's memories, though arc 4, 5, or 6 have their own unique appeals as well. Arc 2 would be the least interesting starting point, in my opinion, since not as much can really be done in that arc with the memories gained. I will not be focusing on what the story would look like starting at arcs 4, 5, or 6, and I definitely won't be focusing on arc 7.
The problem with going for those later arcs is that you miss out on a lot of things that could go differently early on, particularly the events that occur in arc 3. Petelguese cannot be saved in a later arc if he was already killed in arc 3, for example.
I'll now start referring to Subaru as Flugel going forward.
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Going with arc 1 allows you to change the story from the very beginning, and take a vastly different route from canon as well. If you go with arc 1, Elsa can be taken care of early on, alongside Meili in arc 2. Roswaal's plans can also be derailed pretty easily, so there's also that.
Flugel should start with no authorities, as all the sin archbishops have his authorities. In a way, this story should be about him reclaiming his old authorities and reviving the moderate faction of the witch's cult. Since he already knows how to use the authorities from having had them in the past, he should be immediately a master of his authorities once he regains them. The order that he gains them should be different from canon, and it's up to you whether or not the authorities do different things than in canon now that he's Flugel.
I'll explain more of how the story should work with my explanation of if you start at arc 3. Sorry, this is probably quite a mess to try to read.
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Starting with arc 3 means that Flugel still has to get rid of Elsa and Meili later on. Now, if you're starting here, I'd prefer if it's after the white whale's defeat and near the end of the loop where Subaru wins in canon against Petelguese. As in, before Julius and Subaru fight him, but also at a time in which Flugel can call off Julius to make sure he doesn't interfere with his new plan of saving Petelguese. Flugel should feel conflicted on whether or not to save Petelguese, due to his experiences with him as Subaru, but he should decide to put those feelings aside to save his old friend.
Everything should go the same as in canon with the battle against Petelguese til near the end, disregarding the inner monologues of Flugel. At the very end, instead of trying to kill him and having him come back and burn to death, Flugel should let Petelguese into his mind to restore his memories. How this would work is that, basically, Subaru would let Petelguese see his memories, reminding him of who he used to be, and while this is happening remove the witch factor from him somehow.
Since just reminding him of who he used to be might not realistically be enough to restore his sanity, maybe give Flugel an authority that allows him to calm someone down. This would allow Petelguese to process his emotions regarding Fortuna's death, and Flugel could comfort him at the same time. This, in addition to being reminded of who he once was via Flugel's memories, should realistically be enough for him to regain his sanity. Returning his appearance back to how it used to be is another matter entirely, so I'll it up to you to decide what to do regarding that.
While that is a realistic solution to some extent, if Subaru is gaining the memories of Flugel right before he fights Petelguese, there's no way he'd be able to acquire such an authority in time to save him. As such, maybe just say it's a type of magic that Subaru remembered from his time as Flugel? I'm not sure how to solve that issue.
Moving on from that, since Julius has been called off from the fight, (this is due to Flugel saying that he's come up with a better solution to defeat Petelguese than the previous plan), no one is there to question Flugel and Petelguese's relationship. Flugel explains to everyone that Petelguese's sanity has been restored, and everyone is doubtful of his statement. As such, he takes an oath sealed with a curse, (like what Roswaal did at the end of arc 4), that he will not harm anyone with malicious intent, or for the sake of malevolence. Everyone reluctantly accepts this, and the battle is over. Flugel also now has invisible providence, but stronger and with more hands, since he's Flugel and all.
Rem and Crusch are still attacked by Regulus and Lye, and Flugel is upset by this, but not by as much as Subaru is in canon, since he's more experienced with loss and the like. Flugel explains to the Emilia camp that he has regained lost memories of being a person named Flugel, but he doesn't explain that he is the sage, because he doesn't trust Roswaal, in addition to the fact that he knows that the Emilia camp might accidentally reveal it to people even if he asks them to keep it a secret. Flugel convinces the Emilia camp to accept Petelguese into their ranks, too, though looking at him should give Emilia a headache, for obvious reasons if you've read/watched arc 4.
Flugel and the others eventually go to the sanctuary, and from here on I'll only loosely give you a plot to work with. Roswaal is completely oblivious to Subaru having regained his memories of being Flugel, and since his book didn't tell him about this divergence in canon, he is caught completely off guard by Subaru claiming to be Flugel. He decides to proceed with his original plans, but he should be more cautious than in canon.
Flugel originally thinks he should take the trials himself, since he's already accepted his past, but as it turns out, he hasn't. He takes the trial of the past, and is confronted with him having abandoned Shaula at the Pleiades watchtower. After resolving to visit Shaula after the trials and bring her along with him in his new adventures, he moves on the trial of the present.
Here he's confronted with an unthinkable present with all of his old friends living out their lives together, uninterrupted by Pandora. (The idea here is Flugel has some kind of past with Pandora, and that she caused Satella to wreak havoc on the world. Sorry if you don't understand, but I'm leaving this all intentionally vague so that you can insert your own theories into the story.) Reid Astrea, Volcanica, Satella, Petelguese, Shaula, and more should all included in this unthinkable present. Also, Flugel should have a history with Echidnaa be the other witches, so that should make the witch's tea party more interesting, especially when Satella appears.
During the unthinkable present of Flugel, other stuff has been happening, of which I won't get into since I want you to come up with it yourself. Point is, one thing leads to another, and Emilia takes the trials, Otto exists, Garfiel joins the group, Roswaal's plans get derailed, Beatrice becomes contracted… basically, although there should be major differences in how it goes and how it gets there, arc 4 shouldn't be too divergent from canon in terms of the major plot points. Some major ones can still be changed, but not the really major ones, if that makes any sense.
In the one year between arcs 4 and 5, Flugel travels to the Pleiades watchtower early, (I don't know how he should get there, though), and acquires a new traveling companion, that being Shaula. I was also thinking that maybe he could have Reid Astrea come along, but I'm not exactly sure if that would derail things too much or not. Volcanica, for obvious reasons, would cause too much of an uproar if they were to go with Flugel. If you want, you can instead have this all happen after arc 5 and happen in arc 6 instead, where it would cause less of a divergence from canon.
This is already way too long, so you can figure out the rest yourself. I'm not saying that to be rude, to clarify; it's just that I've written over 1,500 words, and for an idea, that is far too long, in my opinion.