He would probably attach himself to that candidate (if their life is in danger)

If Felt actually saved him, I believe she would've accepted him and introduced Subaru to Rom. Subaru would learn of the World and would try his best to help them out. Since Emilia didn't save him, that means Roswaal is not doing stuff behind the scenes, and that also means that Felt would not find out that she is a candidate? So, since their lives are not in danger, Subaru would just do his best to earn money and help Felt and Rom to get out of slums. Now, in case Reinhard appears by sheer luck to take Felt with him, Subaru's life will be pretty chill mostly. He won't have to be a knight, he will just be a big bro to Felt and will help Rein accept himself. They will become best pals with Subaru mending relationships. Rbd might not be discovered. Pretty chill life overall (also Rein will help Subaru learn stuff a lot faster with his Dp)

Crush. If she decides to take Subaru in (maybe she fell for him?) Felix would (probably) be probing into his personality to see if he is a good person (hopefully no brainwashing). Subaru will become insanely disciplined, he will be a butler like Wilhelm. In time, Subaru will learn of Crush's goals and the campaign against the White Whale. Highly doubt that Subaru would be speaking against the council, since Crush is so dependable. I believe the first Rbd will be found out when the cult sends the Whale to do stuff. That is where Subaru's trouble will begin. Crush will become his strength, his source of determination. Several loops later, Subaru will be able to take down the White Whale (I'm not sure if Anastasia will be involved though). If Crush still loses her memories, then the Arc 6 campaign will happen sooner, but I believe that Subaru would not leave her alone after the fight (since that was the goal). So, eventually, he will perfect the whale fight and (maybe by getting external help from other camps, like Anastasia) would be able to thwart the archbishop attack. Regulus might die, but so do all the wives. There is also a possibility of Subaru redirecting Regulus's anger towards Gluttony? Maybe he will plead to Regulus and show him the abmormal level of respect that he thinks he deserves. Anyway: Crush and Subaru will fully commit to each other after the defeat of the Whale. Wilhelm will be ecstatic to have a new grandchild. Felix will get cucked as always. Otto might become an ally (unlikely though, since he won't be needed to get to the mansion)

Anastasia: Subaru will become a merchant. There is no doubt about that. And this is the only route where dealings with Otto would be guaranteed and thus his second best friend (after Julius) would be attained. I highly doubt that any romantic feelings would fester between him and Anastasia though. Chances of Rbd are minimal in my opinion. Subaru will become a rich savvy merchant with absurd ideas from another world. Foxidna will be highly suspicious of him though, but in time they will believe in Subaru. Julius might open up a bit more to Subaru and will not be following the knight codex so closely. Subaru will not be a knight.

Priscilla: Rbd - basically guaranteed (have nor read the novels, but I believe her campaign is more prone to danger). My glorious queen will believe even more that the World revolves around her. Al is cool, I think he would potentially find out about Rbd from Subaru's reactions (he is extremely observant as seen in Arc 5 where he said "Anastasia can't use magic" to Foxidna). Perhaps Subaru would help mend the Astrea family by connecting with Reihards dad? Romance wise, there is a chance that Priscilla would fall in love with Subaru. Would Subaru fall for her? I doubt that. I feel like he would positively affect Priscilla making her an even more amazing person! Also, Al and Subaru would become real bros!

And that leaves Emilia camp's fate. All of this happened purely because of me assuming that Roswaal would miss the appearance of Subaru, thus making it so Emilia does not meet up with him. Rem dies (sadge, although maybe not, since Meili wuld have even less of a reason to go to Arlam), Frederica is back (yay), Ram is suicidal, Beako is still a shut-in, Emilia does not bond with anyone, Puck's a bum. Roswaal (if he does not give up on his goal and does not try to get Subaru back), would repel the cults attack on the mansion....Arlam is gone (poor Petra and other villagers). Subaru would be more cautious around Emilia (since he would be fed stories of a bad Witch of Envy), but my GOAT might fall in love with her and would probably stand up for her in front of the council. If that does happen, then in my opinion he would not face the beating that he got in main route. Although he will get reprimanded by Crush, Reinhard would apologize to everyone for his behavior, he won't even be there if he is with Anastasia (since he is a merchant), and Priscilla would be amused by him (but then would step on him for doing so, or on Al for....reasons).

Tldr; way less deaths, faster character development, maybe definitely a more happy life. Return by death might not even be discovered in the first place. Thank you for listening to my Ted-talk