Yes, Civ V could have more features, it always can, even after 10 years from now with 10 more expansions.
But, does it really need them? I'm against a third expansion. Oftentimes, relative simplicity with depth > lots of features (see where Civ IV fails hard). Civ V has relative simplicity, but was lacking some depth and options in gameplay. The main complaints I've heard about Civ V will all be addressed by BNW, if everything works out. If it doesnt' work out, balance w/ patches is the answer, not waiting another year for a big-release expansion. If they want more money, I'm pretty sure DLCs will make more money than a third expansion. If you want more civs, there are a thousands mods out there adding every type of conceivable civ. Use them. Expansions mean more features and systems, and frankly, if the tourism end-game and Tradition/Liberty/Honor/Piety opener gets balanced, and science runaway is really curbed so that NC start is no longer always the most optimal play style by the upcoming fall balance patch.... then more features/systems will only mess that up. Espionage/Diplomats still feel like a very underwhelming system, but I would favor a DLC release to pair that up a new civ than a full fledged expansion that adds corporations or whatever else (which would then require more time to balance).