Personally, I look at things from a gameplay perspective, so the question here would be: why should we even have rogue planets? What would they add?
Don't forget that these would be extremely cold and completely dark planets. So, certainly no life, nor any activity on the surface. Flying down to a planet and only seeing a black sphere until you get close enough for your ship lights to make a visible difference doesn't sound like a fun time (for most) either.
Neither does coming across these randomly, when you were aiming to jump somewhere else. Finding rogue planets would either be up to chance, in which players would be annoyed after seeing one rogue planet (seen one, seen them all), or it could be done deliberately - and would be even less interesting for many players than visiting mass code A systems is.
Based on the journal docs, it seems like Frontier have considered adding rogue planets at the start, but eventually decided against doing that. I can see why - see the above.
As for limitations, the main practical limitation of adding any new systems to the galaxy is that it would involve regenerating all of the existing ones. Which would almost certainly cause the majority of the player base to riot. Especially since it would also involve most of the bubble changing (everything except the entirely manually placed systems), which would also break BGS and PP into tiny pieces.
This is also likely why we're stuck with the Stellar Forge still having bugs that have been known since almost the entirety of the game. Fixing them would have broken too many other things.
As for what Ian mentioned, which would be hacking new stuff in, I think that a possibility of adding new systems would be to add a second galaxy generation into the game, then overlay the results to the existing galaxy. However, that would also likely come with its own set of problems - for starters, there'd be the problem of having to check for position conflicts, especially in the galactic core. Sure, it could all be done, even if the likelihood of bugs would be high (and as such, extensive testing would have to be done), but the question is: would it be worth the effort required?
In the meantime, if you wish to look at something quite similar to rogue planets, but then there are extrasolar captured planets in some systems, on highly elliptical and inclined orbits. They aren't entirely dark, but otherwise, they are quite rogue-like (heh) planets. In my opinion, those are more interesting than rogue planets could be, especially the ones that still retained a thin helium atmosphere.
If Frontier wanted to add content specific to rogue planets (I'm not sure why, but let's assume they would), then they could easily decide to go add such content to the extrasolar capture planets instead. Personally, I think that would be the better solution - they'd certainly be more varied than rogue planets could be.