Well, that was the basis of my suggestion. Certain systems where the sentinels are a LOT tougher. More agressive, better firepower, more of them.
They could be called… “Forbidden Systems” or “Interdicted Systems”. Something along those lines. They could be rare - possibly based on the existing unclaimed systems.
You could take the original concept further - say there’s something in the forbidden systems that each of the existing races wants, but they can’t have, because the sentinels won’t let them.
So maybe there are ancient data stores that the Korvax want, and trading opportunities or goods that the Gek want, and the V’ykeen… well, they just hate the sentinels in the first place.
So you find a forbidden system, and you destroy all the sentinels and their depots. And all the time you’re doing this, the sentinels are fighting you, and trying to rebuild their depots.
Once you succeed, you are allowed to build a base and claim the system. But that’s not the end - after a while (a week, maybe?) the sentinels will send a space fleet to attack your base, and reclaim the system.
The routines for attacking bases from space already exist in the game code. Such attacks were a regular, if seldom seen, feature of the original game.
The answer to attacks from space would be to approach the Korvax, the Gek, and the V’ykeen to occupy installations in the system (something they want to do). Once the three races have a presence in the system, they will defend it from attacks from space, and it is safely yours.
Claiming such systems, and installing the other races, could give you a special status with those races, or with the guilds.