So what exactly are you suggesting? Instantaneous jumps? Being able to get from one side of the galaxy to the other in a matter seconds or minutes? How would that produce more interesting gameplay or provide any sense of scale or accomplishment? To me, it sounds like exploration isn't really for you.
The point of exploration is that only one person can explore one thing first. Any explorer wants that feeling of being the first to ever see something. I never heard of this neutron Highway up until this thread. At first it seemed like a tedious task to visit these POI's. Beagle Point. Right. Where the hell is that at and why go there?
But nowadays anyone can go anyplace and the path is trodden. It is like mekka, every muslim has to go there at least once in their life.
I like the idea of decentralized exploration. If you can jump a vast distance you create there a personal exploration bubble. But all the space in between the Bubble and wherever you land is still unexplored. So instead of the Bubble slowly expanding as to signify the exploration out from it, you get a more random approach with hotspots that are 'mined out' in terms of exploration. The spots that have not been targeted are seemingly uninspired places where no one thinks to look. That means there are more places where no one has gone yet and so the player can feel like a real explorer.
I thin Frontier got it right. If you played it you'll remember using Shift key and Arrow keys to scroll to any side in the universe and you would go hundreds, if not thousands if ly's away. And in between all these systems, procedurally generated.
Another option is to leave it to the players to decide how large the universe is. That means you can havea group start a project to build a special structure, and go the way of Stargate Atlantis, where they had a gate aboard the ship.
And so a group will make small jumps outward from the Bubble. And other players can then use that gate as a jump point. Before you can jump far, the jump gate must first be moved there. That way the size of the galaxy is relative to the progression made by the project group and how fast such a gate can travel is a matter of debate. Maybe resources must be gathered, energy replenished and the whole structure moves some ly in whatever direction they chose.
So it is like you cannot travel beyond your technological capability to vanguard a jump gate. You can even build in tiers, so that e.g. a jump gate is coupled for energy to a star. And the farther away you get, the thinner the link is so ton say, the less energy can be replenished and it takes longer to charge or you have to move your gate at 77% energy level and be okay with a smaller distance.
At some point the ratio's will be silly. So you have to couple your gate structure to a new star. And that will take extra resources and calibration and whatever story you come up with.
In the meantime, explorers need to scout safe flight paths for pushing the gate forward. And so the whole project will be a spear protruding outward from the Bubble and if there are many, known space will expand in all directions. In the wake of a jump gate structure there will start colonies, midways points and as such, islands out from where there can be exploration for resources and mining opportunities.
If the game has built in limitations to how big known space can become, you won't end up with 2 hour flights to Sag. A. It also means it depends on time investment and it is a CG.