One of the best Elseworlds stories comes from, honestly, a kinda dumb premise. What if Batman...but he was really SUPERMAN?
Not exactly Bruce Wayne if he was Kryptonian, but instead Kal-El of Krypton landing in Gotham instead of Kansas and being adopted by the Wayne family, and being named Bruce Wayne instead.
Things go pretty similar to the original story from there, with the obvious exception of Bruce discovering his powers the night that his parents died and he bakes Joe Chill alive with his heat vision.
For such a silly premise (it's literally grafting the Superman mythos onto Gotham City and hoping it sticks), the story being told is actually genuinely solid. A lot of this comes, of course, from the writing.
Not only figuring out how these two worlds would collide, but through fusing two such disparate mythologies, finding the profound similarities between them when on the surface, they couldn't be more different.
Certainly a lot deeper a story about Superman and Batman than anything Zack Snyder brought to the table.