If we are addressing a “life altering” breakthrough, the choice would have to be a solution to a major “problem” with our daily existence. Not something we think would be neat, like life extension or cheaper goods, but a real solution to a real existing problem.
The biggest problem with daily existence for most people is Transportation (I think–it is for me anyway). 40,000 traffic fatalities per year, ridiculous (and I mean truly, mind-bogglingly ridiculous) commute times for suburbanites. Precious non-renewable fuels being idled away.
I truly think the next century will look back on the 21st and chuckle about the crazy backwardness of our transportation infrastructure/system.
Don’t know what form the transportation solution will take, but my guess would be personal vehicles that drive themselves (so you can work or entertain yourself during the ride), running on non-petroleum based energy, combined with some sort of mass transit system for long hauls that isn’t cattle-car based like subways or commuter trains. Maybe your personal automated vehicle drives itself onto a giant ferry sled, which then warps into Chicago down the median of existing roadways or rail lines at 200mph.
Capitalism is a pyramid scheme essentially, isn’t it? We must continue to grow our population to maintain our wealth and international position, which means the transportation problem can only get worse as it currently exists. But if a suburb can be viable 200 miles away from its closest urban cluster, than we’re good for another couple hundred years.