I think a Japanese constructor will return in the future ( probably Honda because of their history, but others could join too ).
Volkswagen joining in 2018… I won’t believe that until I see it.
Lotus said they are considering building engines for F1 earlier this year, so they could be one; I read also earlier this year that McLaren may consider building their own engine after the colaboration with Mercedes ends, but I think they would prefer not to.
What if we get in the future a Force India-Jaguar(they are owned by Tata, an Indian constructor) or Volvo(owned by Chinese manufacturer Geely)? That would be interesting, and not impossible at all, given the developments in recent years.