I remember being a bit disappointed with Sid Meier's initial release of Civilization V, but the DLCs have fleshed it out nicely.
My guess is that Brad planed to follow the same path with GC3.
And don't forget, GC3 is a complete re-write in 64 bit addressing mode, enabling it to expand exponentially with how much data it could handle over what GC2 could work with. GC2 (and Civ V) are about at the end of what can be done in 32 bit addressing code.