I'm not against the occasional suspension of disbelief when it serves a larger purpose. But it should be used sparingly; basic laws of biology and chemistry clearly show us, for example, that a completely ice-covered planet shouldn't have a breathable atmosphere or large pseudo-mammalian life-forms running about on it (Star Wars mythology notwithstanding). 'Class M' planets with no life on them are another example of writers not doing their homework- so, just where does all the oxygen in the air come from? And so forth.
The universe is an exotic enough place even if we stick to the realm of what we know about it. There's plenty of room for exciting and mysterious locales that don't violate the laws of physics, geology and biology.
A game should wink at no more than one or two inconvenient real-world facts that get in the way of a good game. My favorite is the speed of light.
300,000,000 meters per second- it's not just a good idea, it's the law!
For any interstellar campaign, a means of travelling between the stars in a reasonable amount of time is a must. But you can't realistically exceed the speed of light. It would certainly be possible to design a game where starships travel at or below the speed of light, but not many people enjoy this sort of game. Since we're going to suspend our disbelief on this matter for the sake of a good game, let's look at a couple of the (mostly imaginary) alternatives.
There are three main variants on faster-than-light (FTL) drives. I'll call them the warp drive, the jump drive, and the quantum drive.
The warp drive (a la Alcubierre) is probably the alternative that is most solidly grounded in science. You still can't exceed the speed of light, but you create a "bubble" of distorted space around your starship, and then move the bubble (and your ship along with it) through the surrounding space. There are many practical problems with creating the warp bubble- it requires negative energy (which exists only as a mathematical abstraction), and a fantastic amount of it at that. Then there is the matter of steering: the course of the bubble through space can only be influenced from outside. Any "realistic" warp drive would therefore require a launching-and-aiming facility, and the poor ship's captain will just have to hope that Launch Control has gotten its navigational figures right. Midflight course corrections will be impossible. "Point and shoot" works well for cameras, but not for space opera. Still, it's a possibility, if only a remote one. Those who want to stretch their disbelief a little further can assume that humans have A) discovered a way to provide the vast amount of power necessary to create the warp bubble (tapping the zero point energy of the quantum vacuum comes to mind, if it can be done....) and B) found a loophole that allows the bubble to be steered from within. What you end up with is something like the warp drive from the Star Trek mythos- and a lot of people are comfortable with that. If that suits you, go for it.
The jump drive (a la Asimov in his "Foundation" series, or the much-shorter-range jump drive of the Traveller game system) relies on hopping around the lightspeed limit by going from here to there without crossing the intervening distance, at least not in normal 3-space. Usually a higher dimension with a different metric is invoked to explain it. It may or may not involve a significant amount of subjective time to the travelers, and their experience of time may not match the passage of time in the outside universe. If you adopt this method of star travel, it's up to you to decide if it's instantaneous, or if it takes time, and what the ratio of ship time to planetary time is. Think carefully about it, because it will have a profound influence on the flavor of the game. This option is wide-open as to its feasibility in the real world; have fun with it!
The quantum drive is really a variant of the jump drive, but instead of one big jump, you proceed across space in a series of microjumps (usually many thousands of jumps per second, so fast that you appear to be moving continuously through space). The trick here is that with each little jump, you cross a stretch of space that is a significantly longer than a beam of light could cross in the same amount of time. Examples in the SF literature and gaming include the stardrive of Poul Anderson's "Dominic Flandry" stories, and the stutterwarp drive of 2300AD. The usual rationalization for this type of drive is that some sort of phenomenon analogous to the quantum tunneling of electrons is happening on the macro scale. It's theoretically possible for macro-world objects to tunnel like electrons in the real world, but quantum physics dictate that you can't tunnel any farther than your own uncertainty radius. The uncertainty radius is inversely proportional to the mass of the system being considered; an electron jumps instantaneously from orbital to orbital because its uncertainty radius is much larger than the electron itself is; your two-ton SUV doesn't instantaneously tunnel through the garage door and appear in the driveway because its uncertainty radius is smaller than the width of one of its constituent protons.
It is also for this reason that 60-kg people don't diffract through keyholes or around corners (outside of the superhero comics, anyway).
Still, this sort of stardrive is eminently playable.