Ladies and gentlemen (and the Instructables robot as well), I am proud to present my fourth ball machine on this site. Retrograde incorporates all the skills and inspirations I have acquired so far. In addition, Retrograde includes some brand new elements!
The ball machine is 3'5" (104cm) tall and 2'9" (84cm) wide. It has about 3,700 pieces, and would have had more aside from the fact that I almost ran out at the end. The machine has seven paths, which will (according to my custom) be dealt with in separate steps. I also used the Stepper Lift, and will be making instructions once I take the machine down, since there are none on Instructables already.
So sit back, relax, and enjoy watching...Retrograde!

Note: Retrograde refers to the term retrograde motion, which is the process that make the planets appear to circle back on their orbits through the sky. I thought the name appropriate, because a lot of the ball machine's elements use circular motion.
The music in the video is Chopin’s Fantasy-Impromptu for piano in C sharp minor, Op. 66, CT. 46 and Mazurka for piano No. 23 in D major, Op. 33-2, CT. 73.

Step 1: The Stepper Lift

For Retrograde I tried several lifts, and finally settled upon the Stepper Lift. Through some careful analysis of I_am_Canadian's Twister pictures, I was able to reconstruct a fairly accurate rebuild. I built it four grey rods high (which is seven lifter mechanisms).
Since I always build on a red rod support structure, this lift posed several design challenges. I had to get the lift to connect to the rest of the machine! I settled on the 4 grey rod height because 5 red rods is roughly the same as 3 1/2 grey rods...which was the height of one side of the lift. I simply put connecting rods across the gap.
The other problem was the width - the yellow rod lift would not match my red rod structure, either. I solved this dilemma by placing the lift at a 45^o angle to the rest of the machine. In this position, it fit perfectly. I curved the entrance and exit paths and voila! I could defy gravity!

Note: My gate mechanism may be a little different from I_am_Canadian's...that was out of necessity. There weren't any really good pictures of it, so I improvised... :-D

knexpert#10829476 created a motor housing and posted a picture in the comments thread.

Step 2: The Path Separators

While these are often not covered, they are a vital part of the machine! One special reason I want to cover Retrograde's path separators is because I invented one of them! I saw Sathothy's "ball lowerer" in his guide to ball machine elements. I liked it, and built it to use as an element. As I progressed, however, I realized it would be too tall to fit well as an element. Then I thought, if it goes at the top as a path separator, it would still fit well. So I modified it into a path separator!
It takes two balls to work. One ball rolls across the "trapdoor" floor, and is stopped by the rod just on the other side of the hanging floor. When the second ball rolls across the floor, it is stopped by the other ball. Its weight then drops the floor and the rod holding the other ball, allowing the first to continue along in one direction while the other is tipped in the other direction by a bracket placed below the separator. It took some time to get the thing to work perfectly, but once I succeeded it looked great! I call it the "Dropper Path Separator".

Step 3: Now to the Paths! - Path 1

Path 1 was actually built in combination with Path 2. Path 2's main element is integrally connected with the main element of Path 1...Kariah's Zig-Zag! As seen in the video, the Zig-Zag delivers enough downward thrust to punch the reset lever for the (in my opinion) epic iris diaphragm of Path 2. The ball then rolls off the Zig-Zag onto the ball return, which I've also included in this step. (I did add a few modifications to the Zig-Zag, including an entirely new counterweight material...a golf ball!)

Step 4: Path 2-The Wormhole

Path 2 has my favorite element - my newly invented "Wormhole" element! I give credit for the basic design to GeekBeam (https://www.instructables.com/id/preview-of-the-current-projectnot-completely-done/). When I saw it, I immediately thought "ball machine element!"
Orignally, I conceived it as lying flat, but decided the reset would be much easier if on its side, as it is in Retrograde. I modified it quite a bit from GeekBeam's design: I made the blades more solid, and made the frame opening more circular. I then mounted the modified diaphragm on a larger frame to match my red-rod tower structure. I then attached two levers, one on either side, to control the opening/closing. One worked fine with a golf ball dropped on it, but the other needed more weight. I then thought of the Zig-Zag, built one, and attached the two as one unit. The mechanism worked (for those who use lighter Knex balls, a ball arm may be necessary on both sides). I then moved the frame about until I got the current configuration, and built everything else from there.
Path 2 also sports a Classic Spiral using white snowflakes instead of yellow connectors (which I think looks even better than the original). The classic spiral leads to a "tire hole", which snakes down to the holding track of the iris (the iris was the first thing built, but the rest was the last path built, and I ran out of room...). The iris opens on a yellow connector path, which curves gracefully back to the ball return.

Step 5: Path 3

Path 3 includes the other reset for the Wormhole. It starts with a length of my Green Flex Path, drops onto Sandroknexmaster's Inverted Ball Dropper (whose length I modified to get slightly shorter), and then propels the ball onto the reset claw of the Wormhole. The ball then falls off onto a path which leads back to the lift.

Step 6: Path 4

Path 4 starts with the first ball in the Dropper Path Separator. The ball rolls onto a slightly modified Knextreme Ferris wheel, then to some pathing which leads to a tiny ball arm, and then drops onto the ball return.

Step 7: Path 5

Path 5 has another of my new elements...a pathing spiral similar to the Inverted Spiral Lift, in element form. But lacking tubing, I tried a peculiar alternative: glow-sticks. They fit perfectly in the connector holes! This element is like the lift, in that it is built in a yellow rod structure at a 45^o angle to the rest of the machine.
The path originates from the lower path of the Dropper Separator, rolls to the spiral, through a "Tire Hole" at the bottom, and onto a return path. Even though the glow-sticks don't light up anymore, it still works for my purposes.

Step 8: Path 6

Path 6 and 7 share Kariah's Spinning Basket from Metropolis...because I found out totally by accident that it can be a form of path separator!
As I tested the machine, I used golf balls. At one point, I plopped a Knex ball on the thing...and it went a totally different direction! It has to do with the ball weights. I modified Kariah's element by taking off the blue rod which normally tips the basket inward, and instead placed two Y-clips on the red rod it would swing past. The basket bumps the Y-clips and tips outward. Golf balls are so heavy that they immediately fall straight down, onto Path 6. But K'nex balls are lighter, and the gentle bump simply knocks them over the top of the red rod, onto Path 7. It was a welcome discovery, for I was running out of room for my elements, and this added much potential.
Path 6 goes through the Spinning Basket, and golf balls fall onto a length of my Green Flex Path back to the ball return.

Step 9: Path 7

Retrograde gets much of its iconic look from Path 7. After popping off the Spinning Basket (see Path 6), the K'nex balls are tipped onto a graceful yellow pathing curve, which I "dressed up" with arches and pendulums. The curve ends on another tiny ball arm, which deposits the ball onto a return track. If the Wormhole is my favorite element, this would be my favorite Path.

Step 10: Additions by Request...

Some of you on Instructables have been very helpful in pointing out areas where a few more pictures are needed (notably the top of the ball return and parts of Path 2). I have taken some more pictures of these areas and uploaded them for the convenience of anyone desirous of building Retrograde. Explanatory notes are included on the pictures.
Many thanks for helping me improve this Instructable! :-D