From the quote in my last post it's clear that there would actually be 3 campaigns (since each expansion pack usually features a new campaign). So, in Westwood's first campaign (which probably wouldn't start off on the Scrin planet like ours), First the Scrin would arrive on earth and there'd be a war involving GDI, Nod and the Scrin. Then, in the Second campaign, the Scrin still not have been defeated and the Forgotten make their appearance, who prove to be also a very great thread to the Scrin (possibly even a greater thread than GDI and Nod). Even after this second campaign the Scrin have not been defeated (possibly they just keep sending reinforcements from their planet or something...) and CABAL makes his appearance. CABAL re-emerges with a cyborg faction (what exactly is meant by that I don't know) and then attempts to conquer both humanity (like before) and the Scrin.
I assume that in this third campaign, the Scrin would be defeated. We don't know by who; either GDI or Nod or the Forgotten. It's even possible it's really CABAL who defeats the Scrin... CABAL might even manage to get to the Scrin planet and in the sequel to Tiberian Incursion we'd see instead of a Scrinvasion, a Cyber Scrinvasion

QUOTE (Ishmael-PG)
The Scrin didn't account for what effect Tiberium would have on some humans (i.e. the forgotten). They thought it would kill all humans off, but unfortunately, some that were exposed survived and actually began to be able to harness Tiberium's energies themselves. They turned out to be humankind's most powerful weapon against the Scrin, since they could control Tiberium in a way the Scrin could not, making them a huge threat. The forgotten were going to become a full faction in the expansion for the game.
I think the Forgotten just might be the key weapon against the Scrin...
Now if you think about CABAL... His personality is based on Kane's, who has set his mind on conquering everything (he's been attempting to do this for as far as we know since Red Alert). All Kane Focussed on was Earth however and until somewhere in TS CABAL felt the same. The Tacitus might have shown CABAL that there's actually a lot more than just Earth. The possibility is that CABAL wants to conquer it all... With this in mind, it seems unlikely for CABAL to keeps his primary base on earth. To me it seems likely that CABAL would at least try to leave earth, take over another planet or at least the moon or possibly he'd build his own space station/ship.