I'm 99% convinced that since they revealed it so early (basically at the end of last season, with Death confronting Jack), the plan for Jack to kill God will fail, probably in ep. 15.18. In ep. 15.19, it will be Sam and Dean versus God, and there's only one way that can happen that fits with what we know about this season, with the last four seasons, and with the whole mythology - and that's both Winchesters saying yes to Lucifer and Michael, and using their powers to beat God. It's the only way to properly end it.