Things don't typically get recognized as a line until someone skis it. I feel like the best way to get first descents is to go further than most, work harder, ski something less obvious, or hit unusual conditions. Look for mountaineering routes, that have some chance of being skied, but require more rope work than people would typically put into a ski line. You might be able to find lines that haven't been skied because they're right next to more obvious objectives. A friend and I were able to confirm a "first descent" a few years ago by checking with numerous local guides and guidebook authors about the line we skied, no one we talked to had ever heard of anyone skiing it, let alone a name for it. There is one famous line that is yet to be completed, but many people have tried; the complete descent of Mount St. Elias (summit to sea level -18,000'). For more approachable objectives, I know there are still plenty of potential lines in Colorado that haven't been skied yet, like the one below (shouldn't be too hard to figure out where that is), you might just have to get a little creative.