This workout is an absolute departure from my usual recommendations. While I'm still firm in my belief that heavy basics build the foundation for any routine, advanced lifters understand that muscle confusion can help kick-start a stagnant program. It's also important to note that this routine, while focusing on reps and volume, also includes a pyramid scheme of increased load to ensure progress.

Once Upon a Leg Day

While training with Hall of Fame bodybuilder Flex Wheeler to earn my IFBB Pro card, we increased all our quad exercises to 20 reps with a reduction in rest times and a focus on stretching between sets. Our goal was to maximize blood flow and lactic acid build up while breaking down as much muscle tissue as possible.
We began the workout with leg extensions. We always used a controlled descent and a brief hold at the top, aiming for four sets of 20 reps with 90 seconds of rest. After extensions, we would begin leg presses or what I referred to as the "meat" of the workout. Initially, the approach – 20 reps with 90 seconds of rest and squatting down between sets – resulted in a drastic decline in the amount of weight I could press due to the blood volume and lactic acid build up.
Often times, the oxygen debt was so bad that Flex would have to help me gain control of my breathing by talking me out of my panic state and coaching me through a series of complete inhalations through the nose and exhalations though the mouth, techniques he learned as a competitive martial artist.
Over the course of about two months, as I became better conditioned, I was able to gradually increase the weight and ultimately ended up pressing as much weight for four sets of 20 reps with 90 seconds of rest as I used to press for sets of 10 with five-minute rest periods.
We followed this exercise with a similar rep-and-rest scheme on hack squats for three sets. We would use a deeper range of motion than the leg press in order to really stretch the muscle to its maximum. Leg press depth was kept to a 90-degree angle to keep the hips from rotating upward which transfers the load to the glutes (and worse, the lower back).
Lastly, we would choose either a one-legged box step-up with a band under the foot and over the shoulder for 20 reps each leg or we would perform walking lunges.
This routine was one of the most challenging routines I've ever performed primarily because I'm conditioned as a fast-twitch/low-rep lifter and the oxygen debt and lactic acid build up was very intense and brought about an entirely different kind of pain that completely robbed my body of energy. This type of training is referred to as deep water training by many strongman enthusiasts who commonly perform heavy lifts to exhaustion.
Needless to say, I achieved my best fullness and conditioning to date and won my pro card as a result of this training.

Rep-Based Reckoning

Use these guidelines to build your own pro-sized set of quads, a la Stan Efferding
Exercise                       Sets/Reps
Leg Extension                4/20
Leg Press                       3/20*
Hack Squad                   3/20
Walk Lunge             3/20 steps
-or-
Step-Ups                       3/10 (each leg)

Rest 90-120 seconds between all sets.
Squat down between sets to stretch working muscles.
*After reaching failure on the final set, lower the weight by 20-30% and perform an additional set of 50 to failure