The Eye of Terror Campaign was a fairly solid victory for Chaos. Sure, the Imperium did finally manage to seal off the breach as much as possible (re: push a lot of the invading forces back into the Eye) and Abaddon failed to claim his ultimate prize, Cadia, but for the first time in that sector of space, Chaos has a secure foothold and now possesses and has fortified a number of planets in real space around the Cadian system. Indeed, Typhus as actually fully 'nurgilised' a planet to rule as his own if I remember correctly.
Of course there's a caveat, only the players who took part or were around at the time will know this. The current 40k timeline actually ends at the start of the 13th Black Crusade (the Eye of Terror campaign) so technically- it hasn't happened yet as Games Workshop only want to go backwards and 'fill in the gaps' these days. Which is a shame as it advanced the fluff quite nicely, in addition to Chaos gaining a real foothold in Imperial Space:
-The Dark Angels and Space Wolves finally 'get over it' as the chapter masters learn to play nicely.
-Eldrad dies, his soul sucked into one of Abaddon's Blackstone Fortresses (IIRC, they're also known as the Talismans of Vaul and are in fact, necron constructs).
-some other minor stuff I can't remember
Oh, and I believe Creed is made Planetary Governor of Cadia.
As for a Heresy Online campaign- I'd really be up for that. However, I know other forums have tried it in the past and they've been mediocre at best- too much enthusiasm to get it done quickly. If say, year was spent actually making everything and getting the system in place, and then enthusiasm was generated for it say a couple of months before it was due to start, I reckon it would work! I could only take part in a player format, too many RL commitments at the moment I'm afraid!