Zimmer should have no business in score the Worlds Finest movie. Since this is the first live action meeting between Batman and Superman, a few composers must considered :
1. The form the team of Danny Elfman and John Williams.
2. With Shirley Walker no longer with us, her co-composers the team Dynamic Music Partners. An update version of the new Batman Superman adventures theme is needed.
3. Christopher Darke. Think of the Public Enemies theme.
4. Shawn K Clement.
I can expect Hans Zimmer to at least have a hand in scoring the next film, but I wouldn't expect him to do all of it. He kind of said all he needed to say (maybe all he had to say) about Batman scoring the Dark Knight Trilogy. That said, yes, I think another composer should step in and sort of add to what Hans did.
I don't think it should be Williams and Elfman. Don't get me wrong, though, I love both of those guys as composers. John Williams' Superman score stands out in my head as definitive as Danny Elfman's Batman score, and it would be a dream for them to work together to create something with that. But not this, for the exact same reason I can't really expect Hans Zimmer to score THIS Batman. This isn't Michael Keaton and Christopher Reeve. Those themes for their interpretations of those characters were breath-taking, but this isn't those interpretations. This is Henry Cavill and Ben Affleck. I just don't see it being a good idea, especially at such a formative time in a big universe like this, you can't have two Superman-centric films with two different Superman themes. It takes away from the cohesion that the studio is obviously going to be striving toward.
HOWEVER, I don't know any Howard Shore off the top of my head, but I could totally buy Hans' Man of Steel stuff coming in with some new Batman themes that someone else is writing. I'd like it to be darker, more introspective, but not quite as downright melancholy and tumultuous as The Dark Knight scores were. I will say, I love what Daniel Licht did on Dexter... I could see a vibe similar to that going for Batman. Something that's bubbling on the edge of ambient, procedural, moody, but very calculated and careful, only dynamic exactly enough to be effective and no more.
My opinion is that the score should BE the character in the ways that words and images can't be. This is why I loved Man of Steel's OST. Even with those five notes that Hans uses, you've got a very distinct Clark's "What's-Wrong-With-Me-Mom?" Humanity that shares themes with Lois and his parents, a distinct Authoritative "Don't-Play-Games-With-Me-General" Superman, and a distinct "I-Have-So-Many-Questions" Kal-El, that ties into the themes for the other Kryptonians.
I think the next film's score should follow the same sort of example. That "Authoritative Superman" theme doesn't really have anywhere else in Man of Steel to latch onto. But if you take that Superman-In-The-Suit-And-Cape theme, add a counterpoint to it, then you've got a Batman theme, that isn't the same in the ways that Batman is different than Superman, but can also be pulled out and used as a theme for Batman in his own sections, and expanded upon at great length in Batman's solo spin-offs from here. The way I see it, that would be one of the best ways to really drive home the DCCU in a way that Marvel's Cinematic Universe hasn't done.
Imagine we get five years down the line and we've got all these specific themes for Superman, Batman, Wonder Woman, Green Lantern, Flash, whoever... and they're all fleshed out in their own films, with their own little tweaks for different aspects of the characters, just like Hans does usually in his films... but all of those different distinct parts can come together and be recognizable in a big, badass, grandiose Justice League Theme or something.
I don't see that happening, but I think that would be amazing.