The premise of this one is quite simple; I want Ainz's group, Kazuma's group, Tanya's group, Naofumi's group, and the cast of Re: Zero that exists within isekai quartet, to react to the Re: Zero anime. Perhaps have the warden be the person who sent them to the school-like world in the first place? Have Subaru struggle against the warden's decision to show his adventures at first, before eventually giving in grudgingly. Have the others be confused, wondering why he's so against the idea, and then have the Re: Zero cast be set immediately after arc 4; I know that the isekai quartet anime has them placed somewhere in arc 2 or 3 time-wise, so please do find a way to make that work. You can show the other characters' respective animes to the characters after Re: Zero's viewing, if you so wish. There are multiple different ways to do this however, ways that don't involve a warden or being teleported to a theater. Instead, an alternative option is that for whatever reason the 3 other 'isekaied' individuals, Tanya, Kazuma, and Ainz, have to search through Subaru's memories for answers to a problem they're having. Another option is that they catch Subaru self-harming, and forcibly look through his memories for the reason behind that. What you do with the witch of envy's wrath against them finding out Subaru's secret is up to you, though if you choose the theater option you can just say that some godly abilities of the warden prevent her from seeing what's happening, or something. Just make sure that they retain the knowledge of what happened to Subaru along his journey, and that the witch of envy doesn't make them die or forget about everything at the end.