One thing any novel should do is vary it’s tone throughout. Even a sad book needs happy moments and vise versa.
One episode of Star Trek that did this well is The City on the Edge of Forever. At stake is the existence of the federation itself, if Kirk and Spock fail all the good that the Federation and Starfleet had done would be as nothing. Yet the episode still had very funny moments that are among the most quoted of the original series.
One way this was done was through time. Kirk and Spock had time to decompress, they were not rushing around the entire episode.
This changing of tone meant they could build more tension. It’s more startling if your taken from a moment of hilarity to one of tragedy then one of tragedy to slightly greater tragedy.
The way of the Pilgrim by Dickson is one of the few really good alien invasion stories I have read. It takes place years after the invasion is over. We were against an alien force VERY far beyond us, we were not able to put up any real fight (Think Aztecs verses the US marines of today).
There are two things that really make it stand out, the first the aliens are not just a little ahead of us. They are so far ahead of us that it could very well be possible for one of them with their weapons and gear to defeat the entire military might of the Earth by themselves with little effort.
This is a much more likely scenario then aliens attacking who are far enough ahead to come here, but not so far as to be unable to easily beat us. Think about what a modern air craft carrier could have done during world war two, and that was less then a century ago.
The other thing is that it builds up what would happen to us socially and psychologically during that situation. This is one of the reasons the book is as long as it is, and I feel the page space was well spent. The fact that we do not get a victory through military might or technology makes the ending that much better.
One thing it can be tempting to do is to try and explain everything, to make sure everything is clear. This can be bad.
For example, if I used the phrase, “Don’t be a grammar Nazi,” in a situation where a reader might not be familiar with it I might give a brief explanation, but I could hardly explain what a Nazi was quickly. You could write libraries about what Nazi’s were, what they did and what was done to stop them.
You have to know when the stop, some things you have to leave hanging because they simply are too complicated and too lengthy to go into.
Other things you leave hanging because it helps with worldbuilding. For example in Star Trek it was common for one of the main characters to say “It’s like A, B, or X.” Where A and B were known to the viewers, and X was alien.
This helps make the world feel real, it also can leave you places to build in the future. If you say there was a war between two planets you don’t have to say why it was fought, you can define that later.
One of the more challenging parts of writing anything in the far future is what I call mundane tech. The everyday things that you use without thinking about. Things like washing machines, books, or desklamps.
It may not come up often but you do need to consider how those things will look in your book. If I am reading a book set in the year 3,834 I expect EVERYTHING to be better, not just the big ticket items like power generation.
Once you have a grasp on the big leaps in technology you should just sit back in your chair and look around. Look at everything in your room and ask what you would be using instead if you lived in the world of your book.
This also will let you know if you really do have a good grasp on the basics of your world’s technology. You SHOULD be able to do this, if you can’t then you may need to think about going over your technology again.