To the OP: I like what you're getting at.
The idea of "hyperspace lanes" has always been something I thought would work well and why the Core Worlds never expanded further than they did. Basically, the most accessible/best lanes were found in a certain small part of the Sector and like the rivers of old, connected the far-flung colonies via trade and travel. Within the Core Worlds, hyperspace travel is fast, well-regulated (i.e. protected by the various factions), and predictable. There would be a few hyperspace lanes outside of the core world (in the scenario the OP describing, RNG lanes that may or may not lead to anywhere) but nothing like what the original founders of the Sector discovered when they found the lanes connecting what would become the Core Worlds.
Not terrain per se, but I've always been keen on "monsters" roaming the map, almost as a form of terrain themselves (a la the Weapons of the Final Fantasy series). They create Zones of Denial for most fleets. Now the "monsters" might just be huge [REDACTED] fleets that patrol a system or constellation or other various pirate/[not REDACTED but SUPER REDACTED] kind of foes (unique boss-type ships). Stumbling upon them accidentally might be a rush but having a few story missions or tips tell you where to find them (to either avoid or hunt them) would tie into the campaign. Imagine some spacer in a bar saying "My buddies and I were salvaging near the [X System] and got ambushed by god-knows-what. Biggest fleet I ever saw! We barely escaped with our lives!"
Black/White Holes having push/pull mechanics would also be pretty interesting and add some flavor.
Have we ever though about a Nova/Supernova? I don't know if collapsing a star was within the Domain's abilities but I wonder what a stellar explosion would do to local hyperspace. I doubt such a thing would just randomly happen but a story-driven "race-against-the-clock" mechanic could be fun and if you fail (or win?), you set off a hyperspace wave that fleets can ride.
Oh, and of course, the Gates. Finding active Gate pairs leads to instantaneous travel, which makes certain parts of the sector extremely valuable. Again, story-driven more than likely but it could give impetus for the other Factions to Gold Rush a part of the sector to establish a colony (that you would have to fight/join in on).