(sarcasm and speculation) Did you notice that solders in Earth don't cheer in Synthesis ending even if building are not destroyed? But solders in Earth do cheer in 'good' control ending and 'good' destroy ending. Why don't they cheer when Reaper leave in Synthesis ending? Only possible answer is that these solders were 'transformed' into something that don't think and feel as they did before Synthesis ending. So, What did they become after all? Synthesis is Catalyst's favorite 'solution'. And he is Reaper king(according to him). And It seems that Reaper also don't want to kill these 'transformed' solders anymore in Synthesis ending(Reaper ships just leave). In conclusion : these solders became some kinds of Reaper(husk?) in Synthesis ending. Thus, whole the Milky Way galaxy became some kinds of Reaper in Synthesis ending. Victorious and Uplifting!!! (for Reaper) Now, What will Reaper(and whole galaxy) do after Synthesis ending? Catalyst say that he and his Reaper protect organic life from 'Bad AI' or something. Thus, they should do their job in other galaxy since there are a lot of other galaxy after all; Why should our galaxy so special to Reaper and their king? But in this time, whole the Milky Way galaxy is now Reaper war-machine, since whole life-forms are now small Reaper(or Reaper's cattles). I can imagine that all planets in the Milky Way galaxy will become Giant-Reaper processing ship in Synthesis ending. People will build Reaper 'processing' facilities and walk into these slaughterhouse to become 'super' Reaper(Reaper ships) by their own. From now on, Whole galaxy 'unite' under Reaper to invade other galaxy!!! Victorious and Uplifting!!! (for Reaper). Now, vastly reinforced Reaper armada can invade other galaxy; for example, they can reach Andromeda Galaxy within 400 years. Unfortunately, people in other galaxy don't know about Reaper. therefore, Reaper can secretly indoctrinate organic life in other galaxy and hack their AI to fulfill their prophecy "Creation will always rebel against their Creator" So, they can instigate whole civil war in other galaxy and conquer them by dividing them. Once they conquer other galaxy, they can set their trap - relay, citadel and so on - and continue their genocide('cycle') until other 'Shepard' in other galaxy will choose 'synthesis' solution for Reaper. I feel very sorry for people in other galaxy after synthesis ending. "prepare yourself for the arrival" Synthesis ending = Reaper's complete, total, ultimate victory. I suspect that Bioware was indoctrinated by Reaper!!!