I'm certain that Subaru was Flugel, and I even have a theory about it.
Let's start with some established points:
-both Japanese with extensive knowledge of the stars:
- the same "smell";
- depicted, based on what little we know about Flugel, (in the ending anime and tarot cards) in a similar way;
- both always on the verge of death even though, according to those around them, they never die;
- Hoshin in Isekai Quartet seems to recognize Subaru.
These are some, but in reality there are many more.
Before going any further, I'd like to address one of the most frequently reported counter-arguments: Subaru hates Flugel.
Well, I don't see anything strange about that... Subaru has always hated himself since the beginning of the story; it's the central theme of the Third Arc. F
urthermore, we don't know for sure exactly how things went with Shaula, and therefore whether Flugel wanted to abandon Shaula for 400 years.
Additionally, if it were intended, it wouldn't be the only case where we've seen Subaru come closer to becoming a Monster rather than the hero he is (or tries to be) in the story: The If Story.
In these stories, Subaru's path diverges completely, and in quite a few of them, he becomes an anti-hero or even an antagonist whom the Subaru of the original timeline would hate to death.
This brings me to my theory, and to start with, I use a phrase that I think is quite fitting:
"Memory defines a person."
In my theory, Subaru wasn't actually teleported to our world at the beginning of the story, but rather 400 years ago (or even earlier) to later become Flugel (This would also explain how Shaula is able to understand contemporary terms.)
But how can we explain his return from "Zero" to the present era?
The explanation lies in the way Od Laguna and the memory corridor work.
Od Laguna cleanses a soul of all its experiences and then releases it back into the world to initiate a cycle of reincarnation. This is a law that applies to everyone, but what would happen if an external element were introduced into this system? It would create a Bug.
Well, Subaru, in my opinion, is that Bug. He arrived in this world like Subaru Natsuki, with all his baggage of experiences. At a certain point, after becoming Flugel, he reached a definitive death and his soul ended up in the cycle of reincarnation... but Od Laguna couldn't "purify" all his experiences, only those he lived in the world of Re:Zero.
So a "glitch" occurred, which instead of reborn a new person, recreated Subaru exactly as he was when he arrived in this world (gadget attached).
For Subaru, in the world of Re:Zero, his Zero Point will always be Subaru Natsuki because Od Laguna cannot erase the experiences he had before being teleported.
Or at least, that's how I think it went.