How about an Ahamkara or whatever they're called one? Turns out theu AREN'T extinct, guys. "Forgotten Beasts". Or how about one focused on The City? A breach in the Wall perhaps? With some things to do below? "The City's Seige". Maybe something about the outer reaches of the Solar System? Taking our ships past the Reef to settle new bases? "Light Beyond". Or maybe, we could stumble upon a Cabal portal. After fighting off their initial force, we find it's wormhole to another system entirely, one the Cabal have an important planet in.