I have been thinking of writing a Science Fantasy novel, basically a world with both magic and not currently invented science. I have concerns that it might just have too much fantasy, too much extraordinary, so the world would seem stretched, unbelievable, unrelatable. The mixing of extraordinary magic and science would overload the reader, making them confused and unable to tell what is happening.
Do you think this would be a real problem?
You have a very complicated world, with LOTS of moving parts that will require tons of explanation. I love worlds like that, but you'll need to consider what you're trying to accomplish with the story. All those hundreds of shiny pieces of information are a draw on your reader's attention. If they're too busy looking at the shiny worldbuilding detail, they won't see Carrie's emotional pain at the death of her cyber-familiar.
So you have the whole world, but only a certain percentage of the stuff REALLY needs explanation. It's enough for now that the bartender is four-armed. It's color. But the really critical point of describing him is as a good listener. The waiter has a cybernetic hand he lights the candles with, but the key issue is that it's a military-grade arm and it reveals he's actually former special forces.
Figure out the central point you want your story to make, and concentrate your details to emphasize those parts. If the magic system is critical to story development, go into the messy details of that at some point (preferably revealed in snippets as the story progresses). Pick a few details to play up, and really explain those, integrating them throughout the story.
But if you have a complex world, and want your readers to fully appreciate all the nuances, you may have to wait for a sequel.
The key to writing speculative fiction is comprehensive world building. It's quite possible to take a junk-yard approach, throwing this and that into the mix. And if you do that, it is likely that the confusion that you fear will abound.
The trick with good world building is to focus on how the built world affects the individual characters. That typically limits the amount of weirdness. That in turn reveals the inconsistencies and omissions in the world building. Note that in any realistic world there will be archaic practices that made sense at one time but do not today but are still done because change is hard.
As an example, it is worth studying the laws in the United States regulating alcohol. Where and when it can be sold. What can and cannot be sold. To whom it may be sold and served. A city may have rules that differ from those of the county in which it sits. The county may have rules that differ from its neighboring county in the same state and from the state-wide rules. And so on. All this weirdness takes place in our world and individuals manage to handle it.
One technique that I would suggest is to spread the various aspects of science and magic over time. Start with magic and introduce science. The magic establishment fights progress and ensures that magic must be used in certain areas, even though the science is cheaper, faster, and generally more wonderful. You get a patchwork with lots of embedded conflict. Each of those conflicts can be used to advance the story.
One solution here would be to invoke Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic". So instead of having magic and futuristic tech, your magic is futuristic tech. This way, your readers won't get confused about you having magic and science in the same setting, because as far as the characters are concerned, they're the same thing.
You don't have to go into too much detail as to how this "sufficiently advanced technology" works, especially if it's inner workings aren't essential to the plot. You just need enough of an explanation that readers will understand that it's not true magic, and - like any magic system - you need to make sure its rules are consistent.
I mean, the Marvel films give Doctor Strange's magic a psuedo-science explanation along with some rules of how it operates. And his magical skill is a result of his ability to process information and finite details which he used previously to become a brain surgeon. The universe of Avatar the Last Airbender used scientific principles with it's magic system to a fully integrated effect and many benders were shown to develop specialized forms after studying their element in the natural world (i.e. Lightning Bending is based on principals of electricity. Plant Bending and Bloodbending both rely on waterbending practitioner having an understanding of water's importance to living things).
Star Wars is as much fantasy as it is scifi and the Force and those who can wield it are treated as a magical source. Interesting the films and some of the shows have detailed the mechanics of the force to a greater degree of than the tech and it's mechanical nature. One of the major themes of the franchise is that the force can be used for both good and evil and in expanded works, the nature of the relationship between the Force Sensitive and the Force itself is greatly debated, and two different views could be seen in the creeds of the two largest factions ideologies: Jedi believe the force is something akin to God and that it grants them it's power to act as servants to it's will. The Sith believe the Force as either having no will and thus those that can wield the force do so as a tool, or it does have a will but as a person can choose to ignore that will, they thus have power over the Force by dint of the fact that they can choose.
