Firstly Re: locking capitols. Alioth yes, but Gateway is the home system of a PMF.
They only seem to have done it for Superpower capitals, and not for Powerplay HQs.
(Though Achenar and Shinrarta, of course, are both BGS locked
and exempt from Powerplay)
And there HAD been one NPC faction expansion into Sol.
A couple of factions still in Shinrarta today from the old "drag in by expansion outwards" mechanism. Arguably a bug that the locking mechanism didn't prevent that (though it prevented conflicts regardless, of course) and maybe that's part of why Frontier adjusted expansions to work differently, not to mention some of the odd "home system" effects it caused.
Long gone by the time you were poking around Ross 128, of course.
Which is:
Should Frontier allow real Player Agency?
Should change wrought on the BGS be able to affect lore?
Should we be able to take the game in fresh directions without Frontier spoon feeding us?
1,3) Yes, in principle. In practice is the tricky one. But I wouldn't still be out in Colonia if I didn't think it was both possible and desirable.
2) I think the problem there is the rapid pace of the BGS (Powerplay is marginally slower, but not by much). A heavily contested system could change hands four or five times over the course of a single Interstellar Initiative or Galnet story arc. Incorporating that sensibly into the lore would be very difficult, because the BGS is artificially sped up for gameplay (no-one really wants to spend six months on one election/war)
It's not really very clear what minor faction control over a system means in terms of the lore - or whether it even means something consistent (there's strong evidence for example that it means something very different in the bubble and in Colonia) - and how that relates to superpower citizenship and system allegiance, or to the day to day influence levels in a system. And I'm not sure that it's really possible to make a sensible model for that, either, that fits at all with the lore around superpower governance in the Federation or especially Alliance (the Empire's structure I think could be made to work, though). The BGS wasn't really tied to the lore to start with, and knitting them together now will be trickier.
I think the balance is that if the BGS can affect lore more strongly (the Alliance move into Ross 128, or going back to Powerplay the ALD takeover of Beta Hydri for a while) then they'd also need to put a lot more locks or brakes on BGS activity so that those sort of things were much harder to achieve. At the moment they've been very sparing with that - Lave isn't locked, Ross 128 has permit defence only, AEGIS got kicked out of their home system because it wasn't protected (and that did make Galnet and get an in-lore response).
If they moved to responding to all of those things, I think they'd also need to put up more barriers - things like designating Zaonce or Facece or Tau Ceti as "strategic superpower assets" in game which gives a massive action multiplier to superpower-aligned factions (like the powerplay sphere effect, but much stronger) - to make it much harder to sneak up on them. Whether that would be allowing more or less player agency overall is debatable, of course
It's easier to incorporate on a slower aggregate level, of course - I'll be somewhat disappointed if Hudson's 4-year confirmation vote (just over two weeks away...) passes entirely unremarked or with no criticism from the Liberals about the declining state the Federation is in.