So I'm writing a series that takes place in the final frontier. It's a space opera (well, that's what I'm aiming for) with light science fiction elements. Let's jump forward a year or two and assume my current novel gets published and I'm writing the sequel. I've got a general plot lined our for a potential sequel, and about half way through there is a huge paradigm shift: an alien race appears at our front door. Up until that point the writing is pretty tame, there isn't a lot of outlandish things going on; I don't use lasers or superweapons, there's no teleporting devices, and I try to stay away from common tropes. I guess a similar thing would be Battlestar Galactica - a fairly "realistic" (using the word lightly) take on humans in space, everything can be compared to modern day navies and societies. The alien race plays a HUGE part, it's not something I'm throwing in for the sake of it, or even to "shock" the audience, their role and the characters from this race would arguably become more important to the plot than the humans. The problem is, I feel that if the first novel had absolutely no trace of alien life, it would be kind of weird to introduce it half-way through a sequel. Almost like a betrayal of the atmosphere, if that makes sense. Thoughts?