I believe that before the final arc the big bad will give Subaru a chance to bring back one person who was lost on his journey since I believe the big bad’s Authority allows him/her to affect the story path. This would be why Melakuera in The Frozen Bond OVA said that he faced world-ending crises countless times, and it’s ’cause the world is stuck in a cycle of being destroyed and reconstructed due to the actions of an ancient selfish hero, and the big bad used his/her Authority to make the world highly biased to repeat the general story path the Hero of the world at the time went through when the Seal was likely opened with the Heroine of the story becoming the Witch of Envy in order to stop what comes out of the Seal only for the Hero to not put her down after she becomes a world-ending threat when the threat from the Seal is gone.
I believe that due to the locked-in story path, the big bad cannot meet Subaru much until they are destined to meet late in the story, and due to the big bad having won hundreds of thousands of times or even millions of times, when he/she sees how physically weak Subaru is compared to past heroes, in an act of hubris he/she will give Subaru the choice to bring back a person, who turns the tables on the big bad and shows that perhaps the fucked up story path might be heading in a different direction ’cause of Subaru overcoming his character flaws unlike past Heroes of the story. The big bad would be able to do this due to his/her Authority allowing the guaranteed return of any one person since that Authority is largely responsible for this warped cycle to be present in the first place. The villain would do this since he/she sees no way for Subaru to win since he/she expects for Subaru to be like all the other failed heroes who never overcame their character flaws.
Whether that person outright died due to mortal injuries, had his/her soul destroyed, was sealed, never to return, or taken out of the story through some other means, I believe the big bad will give Subaru a chance, and whether it be through instinct, intellect, or a combination of both, Subaru will choose the right person for the job.