ED supercruise flight mode but extrapolated for interstellar and intergalactic flight. I know it is not scientifically accurate and not suggesting it for the main game but for planetarium exploration with Spacecraft it would be cool I think. Here is how I think of it:
1. Near surface / inside atmosphere - disabled obviously
2. Near planet orbit - disabled probably
3. Medium / high orbit - enabled and limited to sub-luminal velocities e.g. from 30km/sec up to 1c (depending on distance and strength of gravity source)
4. Interplanetary travel within system: Superluminal 1c up to 1000c
5. Interstellar travel: 1000c (or 1kc) and up to 1Mc
6 Intergalactic: 1Mc to 1Gc
Speed will be restricted by gravity field similar to how you travel in ED (ok if you haven;t played ED you wouldn't know what I am talking about
). In ED max speed is limited by gravity of star system bodies. The larger bodies (i.e. stellar mass bodies) have a longer but smoother effect at much higher distances whereas smaller bodies i.e planets have a shorter but stronger effect over smaller distances.
If you keep your warp throttle within 10% - 70/75%, it auto adjusts the speed as you approach gravity sources and you don't crash on them violently. Ok you don';t crash on them even when throttle at 100% but you exit supercruise violently upon hitting a barrier gravity wall, thereby damaging your ship.
If you move really really far away from star system, speed caps at 2000c. In SE it can continue to increase depending on local stars gravity. Within our stellar neighbor it would be around 1kc-1mc. If you fly close to the core / dense areas of galaxy it should hardly reach 1Mc. If you start moving away from the galaxy / or galactic plane where star density diminishes, you could fly > 1Megac and as you get even further into intergalactic space it can go to a max of 1 Gc (one giga c or 1 billion times the speed of light).
I know it is not realistic but it would really help when trying to explore the universe with a spacecraft. Also I have proposed this in the old forum so I am sorry for repeating it here.