Well that just makes it even worse. I mean, there's ending's that are ambiguous, endings that are straight up bad for the main characters...and there are endings that are straight up cliffhangers.
Character gets dragged off screaming "it's a cook book." Sad ending, but it doesn't need anything else afterwards. It's a satisfying ending, if a bit of a downer. You may not actually see his fate, but you can pretty much assume he's screwed. No more need be said.
Character finds the remains of the Statue of Liberty on the shoreline and realizes he's been on Earth this whole time. Also a bit of a downer, but the movie has made its point. It had its reveal. The questions are answered. No sequel needed (even if it did get a ton of sequels).
Woman escapes the bomb shelter of a murderous psycho and finds out in the last ten minutes that there actually is an alien investigation. She could go find sanctuary or she can go help aid in the resistance. She turns to go to the resistance. Okay, then what? In a vacuum, not having seen the original movie before, would may have only just found out ten minutes ago that this is an alien invasion. Hell, even those who did see the first movie just found out ten minutes ago that this is an alien invasion and not some ordinary kaiju monster. I think a hugging explanation is in order!
We think we're following a Event Horizon type space horror only for a kaiju to appear in the last 5 seconds. Did the escape pod survive? How goes the human resistance? Is there a human resistance? I mean, damn, movie! Don't just stuff a kaiju in at the last 5 seconds with no explanation and think that's a good place to end it!
If you're going for an anthology, it's okay to end with a downer ending, but don't end on a cliffhanger with no expectation of resolution!.
Man! Hug this series. I'm moving on to Saw.