We already have the perfect trilogy! A3, despite its production flaws, is the PERFECT ending. It's conclusive, poetic, unapologetic and mature. It reminds us about the indifference and harshness of reality presented in ALIEN after the heroic Hollywood ride in ALIENS. It's full-circle. It's Ripley's story.
A:R is its own movie, but I personally feel that Ripley didn't need to be in it. They could've come up with a similar story, or something completely different, without negating Ripley's sacrifice. Just place the story further on in the future and her sacrifice by default saved several generations of people, which is not so bad if you ask me.
Even so, I fully respect A:R and have come to see it as the new chapter no one dares or wants to continue because they're still hopelessly stuck on ALIENS.
The little we know so far about Blomkamp's A5 mostly sounds like apologetic ALIENS necrophila and indirect franchise cannibalism. Trying to make the perfect trilogy by undoing it is a very odd strategy. Then on the other hand if "perfect ending" means generic happy Hollywood ending with a red bow on top + cheesy openings for even more derivative sequels, then I guess he (or any director for that matter) stand a really good chance.