I'm torn on this. I think Bruce and Diana's moments together were the best parts of BvS. I feel like I want Bruce as the cameo just as an extension of their interaction at Clark's funeral. However, I feel like it would just come across like Tony telling the entire Iron Man 3 story to Bruce Banner. Somehow, that kind of confiding doesn't really seem like this version of Diana's style. Which also means a big no to her basically telling her story to Lois in any capacity.
The only other Justice League member who makes sense would be Aquaman, given all the talk of an Amazonian/Atlantean conflict. However, I think maybe other Atlanteans would be a better fit, rather than Aquaman himself.
Ultimately, I'd say Bruce if it has to be another Justice League member. My personal preference for world building cameos would be maybe James Gordon or Swanwick (as a backdoor to having Steve Trevor brought into present day). Another cameo I would relish would be Alfred. I think he and Diana could really have something special in the way of a scene or two together.