I have an absolutely terrible idea to add to the supercruise mechanic, which may or may not take account of the physics of FTL travel (by that I mean, I'm not sure whether this would necessarily apply to the spacetime compression mechanism). And it basically amounts to an extra overlay of effects and making it slightly more dangerous to travel like that.
First things first, I don't know enough about the way the game handles supercruise's interaction with gravity wells, so I'm going to leave the acceleration and deceleration rates alone for now. Suffice to say, they'll be the same as they currently are.
But achieving a sense of speed when travelling faster than light, in a scientifically plausible manner, aside from the rapid approach of astronomical objects is mostly in the visuals. One thing in particular happens when you travel close to, at and beyond the speed of light - blue shift.
All the light that is in front of you, travelling towards you, is blue-shifted to a point where realistically all you would see is the ultimate in tunnel-vision with a tiny circle of visibility surrounded by a huge cone of just blue blurriness. Behind you would be completely black as no light could reach you.
In terms of gameplay, obviously that would be very restrictive, but how about a mild version of that attached to acceleration? So at 1c, a slight cool colour cast starts to appear over all the light sources, and this cast increases in intensity very gradually as you go faster and faster, until you get to the point where you're travelling at trans-Hutton speeds and the outside becomes just a swirling mass of blurry blue hues, with only the HUD to guide you.
The other thing that happens, because of Relativity being what it is, is that all that blue-shifted light (all the way out to the longest radio waves) becomes ridiculously compressed gamma rays. Absolutely deadly if you don't have adequate shielding. And, let's face it, the universe being the way that it is, compressing all wavelengths of light into ultra-high frequency gamma radiation, would probably be something there can be no adequate shielding against. We're talking frequencies of gamma radiation that do not naturally occur anywhere in the universe.
So, gameplay-wise, we remove the top-speed for supercruise altogether. No more 2001c, and we increase acceleration rates between long distances, BUT with the caveat that travelling at such high compression factors rapidly eats away at your hull integrity, so you have to balance whether you get there at "normal" supercruise speeds (with normal supercruise travel times we're currently used to), but unharmed... or you can push it to the absolute limit, going like 7000c and risk having the gamma radiation rip your ship to pieces before you get there. People may only want to try this if they have adequately stocked AFMUs, hull-repair limpets, if their destination is a starport or if they are planning on visiting a starport soon.
The OTHER other thing that happens is that, when you drop out of FTL - disengaging the warp field - spacetime springs back into place. Not only would this have a noticeable effect on any gravitational wave detectors, but... also fling the accumulated ludicrously-high-frequency gamma radiation caked on the compression front, forwards at whatever is in front of you.
So, someone doing a daring high-speed, unsafe drop-out in front of a starport, might end up killing all the NPCs flying around and invite a lethal starport response? Food for thought on that one. (I know it would also kill everyone in the starport as well, but... game).
The only problem here (which I don't see as a problem) is that it could be used for ganking... I don't see it as a problem because if someone has the requisite skill to drop out of supercruise accurately within the starport's no-fire zone at a multiple of the speed of light, and kill you with a radiation front, I think they deserve the kill.
Some of these things might make the more extreme ends of supercruise travel a little less monotonous.
