Something came up recently that brought to my mind this question: an NSP area that appears to be around 10 ly in radius. Which rather boggled my mind, but then a thought came to me: how many more of such might be out there, so that Frontier can always say that there are still things not yet discovered?
Only Frontier knows, of course.
There are two types of exploration content out there: procedurally generated and hand-crafted. For the former, it has been repeatedly demonstrated that there exist some things so rare that even Frontier didn't know about them. For the latter, well, of course they know about them, and they often tend to have clues inserted that point towards them.
In the absence of new clues, and pretty much all earlier avenues exhausted, the most likely scenario is that the hand-crafted content has run out. After all, if there were still more big things to be found, then Frontier would likely want them found in this long content drought, so they'd add further clues perhaps. They haven't done, so you can draw your own conclusions from that. Maybe they will with the carrier update, but I wouldn't bet on anything major before the next expansion.
So, what else might be out there is the truly unknown generated stuff. That has been there since the launch of the game, or in the cases of landable planets, since 2.0 Horizons. The complexities of that generation brought us years of exploration, and hopefully, whatever the next expansion will be will also add at least something to the Forge.
Because hand-crafted content can be pretty cool, but will only last so long until all its interactions are exhausted. Then it's back to the Forge.
Speaking of interactions though, I'm still surprised that Frontier has had the spaceborne life in so long, and even hinted at more research in them, and has yet to add any such. They could just do stuff like selling gyre pod scrapings or whatever to stations triggering BGS changes, and a number of people would be all over them. There are far too few interactions between exploration and the rest of the game, so there's plenty of room for improvement there.