If this becomes a "sticky", it might be helpful to break it into categories: airframe, interior, engine, etc. There are *so* many options available for each one that a single list would get Yuuuuge quickly
So here's my list of things that I liked when I built mine:
Rudder pedal extensions
DJM Throttle Quadrant
Andair fuel pump and fuel selector valve (along with reduction in height of fuel pump cover)
1 lb Halon extinguisher mounted on forward portion of fuel pump cover (with racing style "loops" w/ quick release pin)
Custom cover for forward portion of flap actuator, courtesy of Classic Aero (covered in leather to match full Aviator interior)
Custom cover for canopy interior side rails, to cover the rivets, with leather from Classic Aero, to match interior
Locking push-pull cables with laser engraved labels (Red for alternate air, blue for oil cooler, black for cabin heat)
Aerox O2 system
Smokin' Airplanes smoke oil system (slightly modified hoses to use leakproof quick-disconnect fittings, modified cap similarly, so I can fill it using a pump without having to take it out and not risk spilling smoke oil on the interior carpeting)
Really minor things, like I made a metal cover for the upper canopy latch so it looks nice, filled the gap between the two halves of the rollbar and sanded smooth so it looks like one solid piece, etc.
Wemac air vents (yes, the plastic ones from Van's suuuuck)
Latest addition: Aithre CO detector feeding directly into Dynon Skyview system...can't recommend this highly enough. Great addition!
Let's see...engine and FWF:
Screwless spinner, one of my favorite parts of the whole airframe
Van's oil cooler shutter
Clear brake fluid reservoir, don't remember where I got it, but I can instantly see the fluid level every time I preflight and check the oil
High-temp (blue) Adel clamps
Not too much else...most of the engine is stock YIO-360-M1B.
Brakes: braided steel flex line for the "loop" around the gear legs at the bottom
Wheels/tires: replaced the axle nut with the ones that have the hole in them, and the removable bracket, to take that jack that uses a bottle jack and the steel rod into the gear leg (y'all know what I'm talking about
)
Conduit in wings (*with* a string left in to pull more wires if ever needed...not yet!)
Internal aileron stops
Internal rudder stops
Fuel drain port fairings
Fuel vent port fairings
"Rocket-style" vent line coils inside wing root
Capacitive fuel sensors (was stock from Van's at the time of my build, though)
Recently added just because they look fun...the RV Printed Parts rudder clevis fairings
Safe-air1 pitot/static/AOA lines and fittings...zero leaks anywhere from day one, knock on wood
Sikaflex for canopy and aft window, albeit with Hylocks where the screws would have gone (to avoid having to fill holes, and they look like rivets from the outside
)
SkyBolt canopy fasteners
"Hidden hinge" on the oil door (but SkyBolts on the two corners to fasten it down)
Electrical/avionics is anybody's choice, so the only advice there: FOT grounding busses, and *everything* (and I mean everything) returns ground to one of those two busses. Result: no ground loops, zero headset noise.
Instead of a homebuilt (or now Van's-available) ADAHRS mount, I used two pieces of angle between the longerons aft of the baggage bulkhead and mounted mine there.
Van's ELT mounts L (ELT) and R (Dynon ADS-B electronics box)
Clik-Bond mounts of various styles used throughout the airframe for wiring harnesses
And no steps installed. Yes, I carry a small step stool for passengers, and I can get up on the wing no problem (when the day comes that I can't, then we'll see...). It's a small price to pay for NOT having to replace them *when* they crack.
There are lots of little bitty things I did, too, along the way...e.g., IIRC Van's has you remove a couple of eyes from the flap hinges, and insert the hinge pins from the middle and then safety. I didn't do that, and simply put small collets with set screws on each end after inserting the pins. Things like that.