I always thought it was weird that there was no Crisis-type story to serve as an in-universe bridge from the DCU continuity to the New 52 one. I don't really consider Flashpoint to serve as a bridge because the main plot of the story had nothing to do with the transition and just threw in Pandora and the resetting of the timeline completely randomly.
Plus, given all of the plots and ideas that had been introduced that could have tied into this story, not having another Crisis story act as bridge seemed like a huge waste. Below I listed some of the things that could have been used, but I'm curious to know what kind of ideas you guys might have for a story like this.
-Darkseid could return seeking vengeance after Final Crisis
-The multiverse is back, which according to the original Crisis is what caused the need for a single Earth in the first place
-Barry Allen is back
-The Anti-monitor is somehow resurrected