Interesting discussion.
My Ooniverse is—of course—vastly different from all Ooniverses mentioned so far, with some similarities.
First of all, in vanilla Oolite there is no Thargoid invasion on planets or stations. There is the occasional Thargoid that somehow stranded in a system, and can indeed be dealt with rather easily. Their in-system appearance is
not what makes them fearsome. The frightening experience is to become a victim of a misjump and find yourself alone in an arena with four or more Thargoids, with potentially not enough fuel do use your injectors
and finish your jump out of interstellar space.
And with a malfunctioning witchdrive that doesn't let you jump out for two or three minutes anyway.
This is what originally makes Thargoids threatening: a band of ships with turreted lasers attacking you from all sides with no place to run or hide. For me that was definitely an adrenaline-inducing situation in 8bit-Elite, and it's still an adrenaline-inducing situation in (vanilla) Oolite.
The threat to GalCop is
not an invasion, but instilling fear of witchjumps to ordinary pilots, which can potentially bring all interplanetary trade to a halt.
Perhaps (to take up one of the points in the first post) GalCop
could quite easily dispense with the Thargoids
if it had a huge and organized navy. But GalCop
doesn't have a navy. For me the main point of GalCop was always that it's an incredibly low-key organization. It is so little organized that having a navy is simply out of the question. Nobody would even think of it. Instead they depend on the incredibly non-effective approach of every-trader-pilot-is-part-of-our-reserve-navy. Again: that's a reserve
without any main force. As far as my own personal Ooniverse is concerned, I am willing to grant that (more as a PR-gag than anything else) GalCop has built a grand total of 16 capital ships (Behemoths). But 16 ships for the whole interstellar space between 2048 systems do not a navy make.
Thus the Thargoids pose a real threat, mainly because GalCop isn't able to deal with them.
As far as the Federation and the Empire are concerned: indeed, they are
not Elite canon. Whether you incorporate them in your Oolite canon is up to each player. I tend to believe in their existence, but don't know a lot about them. I would suppose that both have a considerably military force and GalCop would be no match for either of them. However, for both Feds and Imps GalCop are outcasts, almost physically repugnant. Therefore neither of the powers would bother to start a war with GalCop. Not having to tolerate the presence of anything GalCop is the best thing that can happen to both, so they're simply not interested.