OK... I'll post a few.
1. Moving Aux PTO handle to left side of steering wheel. After 250 hours, I just wonder why Tazewell doesn't make this standard equipment. Thinking about using a cable control someday to replace my shade-tree engineering, but couldn't be more pleased.
2. Adding an additional cannister mufflet to the Kohler's tailpipe as an additional silencer on the factory muffler. After 250 hours -- no brainer, I'm happy. I wish there was a quieter, affordable, factory alternative but I haven't found one...
3. Loading all four tires with WW fluid and Hydro-Seal. May not be for everyone due to additional stress on wheel motors, but I'm happy. Increased stability on side slopes, plus increased the lift capacity before puckering. Only flat I've had since is when I tore off a valvestem with a stick in the woods. Would've welded up that inside hole, except for #4...
4. Reversing the wheels, ala Fourteen. Again, not for everyone because of potential stress on wheelmotors, but the difference in stability is very hard to describe. I can go on sideslopes so steep that I don't feel comfortable in the seat (even with seatbelt AND armrests) unless I'm holding on to the FOPS/ROPS post... which leads to #5... 250 hours plus, and I may never change them back.
5. Replacing the 12.5ci White wheelmotors with 22.7ci Char-Lynn S series. Certainly not for everyone, because (a) at $1500 plus, it was expensive, and (b) it cut the PT's speed in half for finish mowing, transport, etc. With over 100 hours so far, I couldn't be happier with it for MY needs. It changed the whole nature of the machine -- it now feels and functions like a tractor or piece of heavy equipment. Slower, more sedate, but much much more powerful. Still just as nimble and maneuverable, so for work that doesn't require transporting over long distances you never notice the difference in speed -- just use more treadle. I find myself doing loader work at 75-80% throttle because I don't need any more wheelmotor torque than that, and it has enough torque to push the LM bucket into a pile of dirt and fill it, at that RPM. At that RPM, the lift and tilt circuits function much more smoothly and not as prone to jerkiness -- which is crucial with the mini-hoe, IMO. Meanwhile, I can spin all four wheels (with loaded tires) almost at will, if I so desire. Note that I still mow at 100% throttle but that's about the only time I run full throttle now. Burns less fuel, makes less noise, and as long as I don't lug the engine, no signs of overheating. But, the real thing I was going after was hill-climbing ability -- and it is now a "billy-goat." I'm limited only by the lube system on the Kohler engine -- rated at 25 degrees. It will now spin all four wheels climbing a slope, and I sometimes find the need to slightly lift the brush-cutter to lessen the resistance of pushing it uphill and to gain additional weight and traction on the front wheels. Very, very pleased for my needs -- but clearly not a mod for everyone.
6. Finally, the thumb on the mini-hoe. If you don't have one, you simply don't know what you're missing. You'll find uses for it that you'd never thought of before, whether moving rocks/boulders, landscaping timbers, pulling out vines and saplings by the roots, etc. Esamples: (a) I built a set of stone steps (four risers) out of large natural fieldstone boulders -- in a matter of about 3 hours total. Dug the footing down to subsoil, moved the boulders (some in 400 - 500 lb range, at LEAST) and set them in place all with the minihoe. The only stones I handled by hanc were the small "chinkers" that I used to fill cracks or voids between the large boulders. (b) Moved and installed 8 used railroad ties to replace the cheap box-store landscaping timbers around flower beds. Removing the old rotted timbers, widening the hole for the new ones, then carrying them about 100 yds and placing them took about 30 minutes -- again without handling anything by hand. Note that the aftermarket adjustable, foldable thumb can be moved back to it's stowed position in 30 seconds or so, and doesn't interfere at all with digging...
I've done other mods, such as additional lights on the canopy, steerying wheel knob, solid tires on the front of the brush-cutter, etc. but they're minor in comparison.
Hopefully, this summer, it will get done. 
