Well I just picked Deathstroke because hes a villain that audiences havent seen Batman go up against as of yet and given that one can never be too sure as to when or if Warner Bros. will make a live action Teen Titans film sometime this decade, then I would just have him introduced here since he is a mercenary for hire.
In regards as to why Lex wouldnt just use Metallo for both Batman and Superman, well I had this idea on what this film could really be about. I mean, this is more or less my take on what Id want to see for this film.
This goes back to my idea that Lex could be the type of person who would want to destroy things in order to rebuild them in his image. So after having helped rebuild Metropolis from the destruction that was caused in MOS, the people of Metropolis really look up to Lex as a savior type of figure, and Metropolis has more or less become a city that Lex pretty much owns.
So Lex has larger plans in mind and begins instigating wars and destruction between countries across the globe using Deathstroke as his pawn so that no one would realize that Lex was behind all of these events. This way, Lex can come in and come off as their savior, preventing wars or rebuilding cities/governments, all within his image.
This would go back to what Goyer was saying on wanting to see Superman tackle some real world issues. I mean what would Superman do if people were to start asking him to stop wars and to actually get involved in those types of situations? Would superman go against governments and upset the established order so that he can do on what he believes is right? Things like that could be some great scenarios to place Superman in.
So during this time, once superman starts becoming a problem for Lex, he creates Metallo in order to take out Superman, while Deathstroke is deemed adequate enough to handle batman.