The inner burbs must increase in density for those that rely on feet, bikes or public transport. Zoning and codes are obstacles to density, but they can be adjusted - formally or informally. I'm wondering about infilling between houses. I used to draw townhouses, and the site planners always left addresses for the spaces between row house blocks. I couldn't imagine anyone filling in back then, but I can now.

I'm also wondering which buildings will be adapted to serve as local schools for kids too far from the mammoth schools we've been putting up.

I could see the outer suburbs becoming a spacious paradise for those that can still afford to drive that far. Exurban highways might become less-traveled and convenient for long commutes, but maintaining them will be expensive. Alan would want money for urban rail while Mr. Big would want the highways kept smooth for his Rolls-Skoda.

The advent of suburbia is a result of more land (and secondarily resources) available for a smaller number of people.  Granted, US population has still increased overall, but our surplus of land still remains.  

Your concept of the extended family moving in together is reasonable, but I wonder whether you have taken into account the counterbalance that as it gets more expensive to commute long distances, suburban houses will become cheaper, ironically being a great place to live for those who don't have to commute.  We're not just going to tear the suburbs down.  And neither can current urban environments support all of the people living in suburbia (even if they move in with family members, there are limits).  I guess we could live like people in India or something, but I find that to be unlikely.  

I think we'll see a move toward some consolidation.  Suburbs that are an extreme distance away from urban centers will languish and die (I guess those who can afford to live out there will be able to have a massive estate, rather than a small 1/4 acre plot), but those within reasonable distance 20 miles or so, will not.  Greatly we're going to see increases in transportation efficiency.  We can improve our transportation system by a lot and reduce a lot of our use of fossil fuels.  People haven't even really started to move away from gas guzzlers.  We clearly have much room for improvement.  

Much, if not most, of the new housing in the US is suburban or exurban, but there are already some counter-trends happening around the country. A number of cities, especially along the West Coast and in New York and Chicago, are seeing lots of new high-rise residential towers go up. "Transit villages" are springing up around inner-urban and suburban communities. Some of the mature "Edge Cities", like Hacienda Business Park in the Bay Area and Tyson's Corner in Virginia, are diversifying their land uses beyond just offices to add dense housing, hotels, and the like. Finally, a number of suburbs which have an old-fashioned district are trying to revitalize them by "main street"-type programs. These trends, along with the others people mentioned, could become more pronounced as the oil peak (or the liquid peak) is passed. A traditional virtue of cities, which is to maximize access while minimizing energy and transport costs, may once again shape urban form much more strongly than in the past 50 years or so.

It is true that we have a huge fixed capital investment in suburban buidings and infrastructure, but it is also true that that infrastructure will require huge ongoing investment if it is to stay in a state of good repair. If oil peaks, transportation investments (of whatever is left to invest) could be shifted from highways to reviving rail services, as many of the rail lines still exist. Transit routes could be established, or reestablished, between new nodes.

Buildings, too require reinvestment; Stewart Brand, in his "How Buildings Learn", noted that buildings need a big reinvestment at an age of about 40 years, as roofs, siding, plumbing, and other components wear out. Post-peak, some buildings, expecially the most remote and energy-intensive ones (i.e. offices without operable windows) may no longer be worth the trouble, and abandoned or dismantled to build other buildings. Other single-use buildings may be adapted to new uses, and "grayfields" -- suburban parking lots -- may become the sites of new mixed-use settlements.

Just as one example, the stock answer says you have live close enough to shopping to walk or bike. But outside Denver, where the sun shines and the wind blows, most suburban houses have enough energy available to them if they could capture and store it to run a small, efficient electric vehicle the several miles needed for shopping every couple of days. What technologies make that possible? How soon will they be available? Or you could consider it in reverse. There's lots of energy available on the roof of the Safeway, is it enough to run electric delivery vans to take groceries to the customers who order online?

Another stock answer is that suburban housing requires too much energy for heating and cooling. Ground-source heat pumps are very efficient, are feasible retrofits in the Denver suburbs, and use electricity from any source in place of today's common natural gas heating fuel. Combine that with much better insulation. Is it good enough?

Suppose electrification is possible. Colorado is rich in potential wind and solar resources, has enough coal to meet that augmented electricity demand for decades, and significant uranium and thorium that could run CANDU reactors (thorium needs a bit of seeding); does suburbia survive in Colorado but not in New Jersey? Not to pick on New Jersey in particular, but they seem to have a lot of people and darned few energy resources. Still, is balkanization of the US along energy-rich and energy-poor lines possible?