A couple of options for one of the most talked about movies in 2013, that doesn't even come out until 2015. First of all, the obvious beginning should be centered around the total destruction of Metropolis. Tying Bruce Wayne in, on helping to rebuild Metropolis. Helping to restore faith, possibly showing Superman in a very bad light.
We then could have Bruce teaming up with Lex for some time, until Wayne figures out Lex has nothing but bad intentions. Which should directly lead us to Superman and Batman teaming up to go after Lex and another Villian not yet told/shown for production.
My personal opinion is that this first movie shouldn't over fill the first movie we have the caped crusader vs Kal-EL. A few cameos, and an after credit scene of Bruce, Clark, and Wonder Women in a meeting at Wayne towers. Show them talking to a mysterious figure, then a close up of either Robin,Green Lantern, or The Flash
This is all my personal feelings about where the first movie should go. Take it or leave it, like it or hate it!