Spider-Man: Homecoming - 10 New Things We've Learned (Aug 19th)
6. Doctor Strange's Stinger May Well Point To It
Recently, a tweet by Benedict Wong seemingly suggested that he'd shot something additional for Doctor Strange this month.
That suggests a stinger or a mid-credits sequence that wasn't initially planned. Perhaps it also has something to do with this "scoop" that Tony Stark has a cameo...
The reasonable assumption there is that Stark appears in the Doctor Strange post-credits to set up Spider-Man: Homecoming. Possibly with him and Wong discussing Strange being registered? Who knows, but it's a nice little rumour.