Yeah, I think it's asking a lot for 2 more Alien films from Ridley Scott, I mean he will make other films in between as well.
The next Alien instalment, if it's to happen at all and as quickly as possible, will out around 2019/20. Then the one after that, if it was to come out as quickly as it is feasibly possible, would be 2022/23. Ridley Scott would be mid to late 80's by that point.
What I'm saying is that there is only one more fully Ridley Scott Alien film to come, if that at all.