So armed with some science from the video above, can you create some enticing yet plausible alternatives to supercruise as the primary means of interplanetary travel?
The whole reason these manifolds are a thing in reality is because the gravitational influences at work have protracted periods of time to act on other bodies and are traveling at significant velocities themselves. Reduce travel times by many orders of magnitude and/or increase acceleration/total delta-v by many orders of magnitude and you radically diminish the relevance of the phenomena described, even if you are obeying the same overarching natural laws.
I was thinking more along the lines of visual cues, but following these gravitational / transport network "routes" could permit faster supercruise travel due to "favourable gravitational conditions"
We already have that, to a degree, with mass shadows. It doesn't apply much to really long SC voyages, and couldn't, without a much longer range of more pronounced gravitational effects and greater SC acceleration. I think this would be very hard to balance to the extent that long SC voyages were potentially shorter and more interactive, without crippling SC travel in closer proximity to sizable collections of mass.
In other threads on fast travel, I've actually suggested supercruise slingshotting, I even made some diagrams for it where the green section fo the gravity well is for slingshotting and the orange section is when you "fall in" and gravity brake
But why would SC behave this way? What causes that cut off between the slingshot and breaking zones?
Even if gravity could act on ships in SC in the same way as it can on real objects in real space, gravity assist could not work, because the relative velocity of most planets--compared to the velocity needed to make interplanetary travel take the same amount of time it currently does--is completely trivial. The system we have now, while also arbitrary, is at least consistent from the perspective of gravity limiting SC velocity.
Not to mention that the current supercruise travel times / speeds aren't grounded on any kind of physics or reality either, they're just some arbitrary values someone at FD came up with, which pretty much invalidates any kind of arguments based on "realism / plausability".
Realism and plausibility are not necessarily the same thing. You can have a high degree of verisimilitude in an overtly fantasy setting...you just need internal consistency. Once the rules of the setting are laid down, anyone that understands them should be able to use them to make predictions on how various phenomena in that setting will interact...unless one is being deliberately surrealistic. Cause and effect, at the micro and macro levels, should be logical, even in distinctly unreal settings.
FTL, especially as it's depicted in this game, is as high-fantasy as an undead dragon with a lightning-bolt breath weapon. That's still no excuse for SC to behave in self-contradictory ways, IMO, unless there is no other alternative. And there are plenty of potential ways to speed and/or introduce more gameplay to travel, while improving consistency, rather than further compromising it.