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?
I wouldn't worry about the sequel at all. Make the first book good, whether that requires aliens or not.
I have wrestled with this same question quite a bit, and so my answer may be strongly influenced by the way I decided to handle it. But, I would say that you need to figure out how the aliens appear to decide whether the timing is realistic or not. Specifically, you need to know what kind of technology they're using to get around. Something that kept coming up as axiomatic in the research I've done on this is the idea that "there's no stealth in space". A spaceship with any kind of life support/computer/whatever system running would be so hot compared to the background of space that, even with its engines off, you could see it coming from across the solar system easily. With its engines on, you could see it all the way from the next star system over. If it has really big engines (which it would need in order to cross interstellar distances in any reasonable amount of time), you could see it coming from light-years off. This is predicated on a lot of assumptions: namely, you're constrained by the laws of physics as we know them, which means no perfectly efficient machines (i.e you will always have waste heat) and no faster-than-light travel, and your spaceship is using an impulse drive of some sort (i.e spitting hot gas or plasma out the back). If you change any of these assumptions it may be possible to sneak up on people; for example if you have a reactionless drive (using gravity waves of EM impellers or something), or can teleport, or can dump your waste heat into an alternate dimension or something. Anyway the point of all this is that your first encounter with aliens is either going to be, "We've noticed a sudden large burst of really hot plasma near a star 30 light years away, we're pretty sure it's artificial and headed towards us!" (and then wait 300 years for the aliens to actually arrive), or it's going to be everyone sitting around at lunch and suddenly flying saucers descend from the sky. I think you can imagine the implications for plotting stuff out over the course of a series of books.
Easier said than done, but yeah I've managed to squeeze in a reference or too, albeit very, very subtle ones. While I understand the train of thought, I've never worked like that. I always keep in mind that the future isn't for certain, but I always like to know where I want to go. I'm doing nothing with a potential sequel at the moment, I'm focusing on the current project - but knowing how certain events will unfold and where certain characters will end up makes the process of writing in the present far easier - at least for me. I'm also vigilant when it comes to writing plot lines in a way that they are forced into future plot lines. I treat everything as its own entity, and leave its evolution for later. That's actually a very good perspective, one that I hadn't even considered but seems obvious enough. I'll have to look into this more.
Well, let me know how you end up doing this. I've got a story that begins with aliens showing up at Earth in the very-near-future, and clearly I've been struggling with how to do it for a while. So if it isn't any trouble it'd be great to hear a fresh take on the mechanics of it.
I think it would be weird for aliens to randomly rock up in the sequel. I think readers want to know what they're in for. Like DeathandGrim said, you want hints of it. Could they come across something that's half-alien? Some signs of life or the potential for life like we do in real life? Set up the kind of atmosphere that makes it feel as if no-one is ever really alone, even out there? Kind of like that weird chilly feeling you get when you feel like ghosts are around? Just throwing ideas around.
My MO trilogy has aliens introduced in the first novel, via an inciting incident towards the end of that novel. The aliens are stealthy or rather cowardly, opportunistic bastiches. They are not interested in full-frontal assaults or initiating direct conflict, seeking to manipulate (in this instance Earth's inhabitants) indirectly via advanced technological means. You could possibly use a similar device, where the effect of the alien species is made known, without direct reference? Cows disappearing, for example?
This is what I'm thinking. There could be an obelisk of unknown origin, or electromagnetic signals from deep space that don't fit any pattern. However, you'd need to be careful to not mislead the reader into thinking that they're going to get some sort of closure on these mysteries in the first book.
Also, FWIW, I am with you in planning out the novels in advance. Very much a planner or plotter when it comes to story, and I have a few layers of the story mapped out over time covered by each of the three novels, evolving from novel to novel, as well as the catalyst for a fourth that breaks the mold entirely.