"see the person that you first met" aka his twin sister, as she was in the womb with him and would be the first human being to come in contanct with him. "return to where it all began, reclaim what is rightfully yours" aka go back to your family, those that colluded behind your back to kill you and reclaim your position as the heir of the family. return to where it all began, is about where his bad past started, with him being forced to go into the battlefield and leave the family. as for the rest, well he said it himself, it was about lu sheng. 

honestly, it was easy to figure out this much (other than the twin sister bit as we did not know of that prior to this part of story) but everything else we already got a glimpse of.. 

seeing how regions are merging already, i'd say that all of earth was already united in round 20 he** maybe even other dimensions were united with them which we will not know till later on, because already there aren't lots of people alive out of the ~2b people that were taken. 

also from the start we already saw that the angels were malicious and they killed without care but even then, the game wasn't designed by them as they are nothing more than pawns, i think the one that designed the game is a divine being or even just the will of the universe.

well, they never specified that the end of the story is at the 20th round, they only said they need to reach that to save earth.

that meaning that later there could be wars with the angels or other forces outside of the rounds themselves.