Alien was largely absent of answers as to where the alien came from, and even the sequels focused more on the alien than the origins. It's only Prometheus that's tried to answer those questions. And the movie wasn't very good!
Rama would work providing the tension came from the humans bitching about Rama while Rama itself is just stopping by for a pit stop, oblivious to the trouble it's causing. There doesn't need to be guns, or splosions, or Rama posing a direct threat. Hell make the biots more menacing but otherwise the story can easily hold its own.