In order to Go Big in space, we need a few things.
1. We need cheaper transportation to space. SpaceX's work with the Falcon 9 & Dragon are promising in this regard.
2. We need to develop what NASA calls ISRD - In-Situ Resource Development. Or in plain English, space-mining & manufacturing, as well as things like building and maintaining a network of fuel & supply depots in space. You can't bring the materials to build an O'Neill colony from Earth. The way you build something so big is to mine an asteroid, get enough metal & other resources to build up there, and send up enough manufacturing equipment to make more manufacturing equipment, until you have enough to build your space colony. Once we've managed to get problem #1 managed, getting the equipment, manpower & science/engineering know-how to do things like mining the Moon and asteroids is achievable. Technologies like 3-D printing are promising in terms of making it possible to custom-build stuff in space. We still have to engineer spacecraft that can go capture asteroids, that can extract metal from them, as well as fuel, water, oxygen, etc., make things like sheet metal, or fiberglass, or other raw building materials that can be made into parts.
3. We need a better handle on human survivability and thriving issues. Keeping people in space for less than a year is within our present capabilities - you need air, water, food, a place to pee and poop, temperature control, and you're good. But then the bones start losing mass, muscles atrophy, cells start getting their DNA damaged by cosmic rays. You do not want to have children in space until you have a space habitat that's big enough to spin for pseudogravity, and has far more radiation shielding than the ISS.
4. Economic reason compelling enough to get the folks with the big bank accounts to start writing checks. There are a few reasons to go into space today - scientific research, communications & weather satellites, political flags-and-footprints missions when the stars are aligned for that sort of thing. But while that gets us into space, that doesn't get us to Go Big into space. The Planetary Resources business plan is promising - they want to mine asteroids for platinum group metals, which are extremely rare on Earth, and there's enough potential money from that business to kick us a few steps closer - for funding things like in-situ resource development. The space tourism thing also promises to kick us a few baby steps closer - it's got enough rich people to write checks that are funding the development of new spacecraft, which may bring down launch costs.
Of course, all this takes a huge amount of money. I think that once we've figured out problem #2, we absolutely have the means to build bigger and bigger, eventually doing things like O'Neill habitats. Assuming, of course, there's a reason to get someone to pay for it.
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