Depends....
For spring 2018, both Boston and London will be throwing money at him to come and race there. Boston will see good competition and if Rupp wins there, he is on his way to becoming another great USA marathoner (but his time would need to be at least a low 2hr 08minutes). But the course would be tougher for him and he might not be able to use his track speed so much. He might need to use his mental toughness to win there like Salazar (see 1982 Boston Marathon, the famous "Duel In The Sun").
If Rupp chooses London, then he can definitely use his track speed and improve his 2hr 09minute time from Chicago. He will probably have a bit more competition at London (not much, but still) so he might have a bit more difficulty winning at London (due to flatter course and no hills to use for help). And he would need to run a mid 2hr 07 minute to be in the hunt for a good showing.
For the fall, Berlin is too close to track season and I think I read that Salazar and Rupp will still be looking at track races to help his speed out. Thus NYC is the place to go, especially if Rupp wins at Boston. If Rupp wins both Boston and NYC, he will definitely be considered one of the best USA marathoners, and looking at strong contention for 2020 Olympic gold medal in the marathon.
Which leads up to 2019. Come 2019, I would suspect Rupp would want to run only at Berlin so that Salazar and he can see where he is at compared to other top marathoners for the 2020 Olympics. But then the 2019 World Championships are Sept 28 to Oct 6. Could Rupp go for the WC marathon, or the 10,000-meter track race? If Rupp is selected for the WC marathon, then I suspect Salazar would have Rupp run track in the spring to early summer. Then get ready for the WC marathon. By running the WC marathon, Rupp can get a possible preview of 2020 Olympic marathon.
If, however, Salazar and Rupp decide the WC 10,000-meter track race, then most likely London is 2018, NYC in fall of 2018, and then Boston in spring 2019. Run the 10,000-meter at WC, run the marathon trials in 2020 and run the marathon at 2020 Olympics......