(Below will be mostly just my thoughts on the series of Re: Creators, so you're free to start somewhere near the bottom of the paragraph if you'd like to just read the idea.)
I don't see Re: Creators talked about nearly as much as it should be. I think the main reason for this is that it's held back to an extent by its own characters; the idea of the show itself is truly what sells it for me. Anyway though, characters coming to the real world from their own worlds is quite an interesting idea that isn't really done usually. Crossovers are usually about sending characters to worlds that have different systems of magic/other things than the original world, and seeing how the character adapts, or seeing how the characters of the new world adapt to the original world's character/characters. Re: Creators does this differently, where they're adapting to the real world rather than a new system of magic. From what I've heard of the series, at least. Either way, that's enough of that. What happens when you put actual characters from other animes in Re: Creators rather than characters made up for the sake of the show? You get a huge crossover. I want a post TYBW Ichigo, a post UBW Archer/Emiya Shirou (your choice), and then from there I want singular characters from other series that you would like to see with this idea/request. Also, this request may be incredibly flawed since my knowledge on the series is lacking, so feel free to correct me on anything I'm wrong about.
Update: I no longer think Re: Creators is held back by it's characters, after reading a certain really well-made crossover between the Fate series and it. I just wanted to amend that statement, and say that Re: Creators is a good story on its own, and I hope I haven't offended anyone by saying otherwise until now.