With any place, I always start thinking about the overall urban fabric and feel of the city. I think the most fundamental change to that is going to be the continuing, possibly expanding, stream of new multifamily projects. Not only will these fill out areas where we are already seeing growth, but in other key nodes where multifamily and mixed-use projects will suddenly seem more viable. We are seeing this already in places like Mueller and the Domain, which proved that there is an appetite in the city for creating new centers of mixed and higher densities from scratch on larger underutilized land tracts. Likewise, East Austin and West Campus have shown that underdeveloped central neighborhoods with established street grids can also actually manifest into walkable districts.
This is going to be key for future growth because it's going to balance the inevitable sprawl in the suburbs that will naturally accompany metro area growth. I feel it isn't so bold to say that most if not nearly all of the strip malls in the central parts of the city will disappear, with corridors of 3- to 8-story mid-rises becoming the norm on the major avenues and boulevards. South Lamar is the most extant example of this now, but North Lamar, Burnet, East Riverside and several East Austin arteries aren't far behind. It's justifiable to predict that most major cross streets in both North and South Austin (Anderson, William Cannon, Koenig, etc.) will follow the same pattern in time.
As others have touched upon, forthcoming large-tract developments like the Grove, Highland, St. Elmo will bleed down nearby major streets transforming them with this new widespread mid-rise density, and connect with the others. I'm almost picturing neurons connecting, nodes and narrow corridors in between them. Of course this will leave some of the established low-density areas in between as enclaves, and some will retrench and new historic districts and downzonings may proliferate, but it would be up to, and behoove the city the push through land-use reforms that allow more "missing-middle housing in these neighborhoods, roughly from 4- to 20-unit apartment buldings. This would allow more balance between the busiest and most quiet communities.
The success of these developments and the spread outward will have developers taking second looks at activity centers and corridors that did not previously demand such density. How about Southpark Meadows or Colony Park, Brodie or Far West, or even the area nearest the airport as it expands. Further, as these hypothetical development hotbeds shed their car-centric images, buildings of 12, 15 or even 20 floors could begin to appear in more established (by then) areas that are developing now.
I think we all can agree the south metro in Hays County will look like Williamson County does now. However, it's not so farfetched to also expect some taller buildings to appear in certain parts of the north metro. Likely, in a few key downtowns, Round Rock and Georgetown, some core corridors, like Bell Boulevard in Cedar Park, and the Lakeline area...as well as around 35. Speaking of freeways, while the major arteries in the city core will stay as they are aside from the addition of toll//HOV lanes, you can bet they will expand them outward and also likely lay down another partial ring road beyond 45/130. I also think the wealth surge with more and more jobs descending upon Parmer Lane from Robinson Ranch to Samsung and beyond is going to up the price point and quality of developments in southern Williamson County.
In conclusion, I've thought about it a little.
Truth is, though, because this is happening now in history, and that this influx of population is mostly millennials, Austin will eventually look strikingly different than any other city in Texas...because what we want from the places we live is vastly different than previous generations. I've always called this city a "millennial darling," with good reason.