Even in the best of scenarios, there's no way that they'll make Batman as minor of a character like how "Venom" (not Eddie Brock) was in Spider-Man 3.
Even I have to admit that Batman in general is just too big of a character to be given that type of treatment.
Using him to the right balance will be tricky, but not impossible, just very difficult. They need to utilize Batman's character where his purpose for being in the story is to help forward it, especially Clark's journey, so that it's less evident that his inclusion into this film isn't just for pure gimmick and a quick cash grab at the box office.
He needs to be in the story where he doesn't end up humiliating the character and making him come off as a lesser hero/being, but in the same time, they can't have superman do the same thing to batman either.
Honestly, I think this film could work better if this were viewed as MOS 2 featuring Batman than a straight up WF film. Why? Because we've just received a freaking successful trilogy featuring Batman and it would have only been like 3 years since we last saw him, played by Bale no less, when this film comes out.
You don't want to overdraw his welcome with the audiences and critics; I mean look how many people thought that it was too soon for a Spider-Man reboot last year.
Plus, if this were to be viewed as MOS 2, you'd satisfy the Superman fans while still capturing enough people's attention that Batman is going to be in this film as well.