Running pretty stock, most changes I've made have been due to minor annoyances, preference or part issues. Have titan + e3d ready to put in, but i'll get a little more use out of the stock hotend before switching.
*15 x 15 easy open cable chain (only fastened with one bolt on each end - seems to hold pretty well), stock chain was frustratingly small and provided cabling in some cases too short! Had stepper cabling go bad in my previous printer, so didn't mind paying a little extra for future easy access.
*The 3d printed gantry parts (thingiverse) to combat the countersinking issue.
*Tamiya connector for hotend (#nosolder - haters gonna hate, but they look okay to me on thermal cam during operation /shrugs), also running better quality silicone coated RC battery cable and ferrules for all my main power/hotend connections.
*2 pin JST connectors over Ethernet pairs for fan power/thermister etc
*Cast 2020 corners here and there - mostly top of frame - built frame initially with them as I was having difficulty getting things square with the melamine, but during the rest of the build took cast corners off that got in the way of other parts.
*eFuel 12-18v 30a PSU - honestly after plugging the stock one in and it going pop, and having 2 previous printers PSUs go prematurely noisy with fan probs I decided to go left of field with a hobby psu. Came to AU$100 after an ebay 20% off sale, seems to run pretty quiet compared to other 3d printer psus I've had (although I don't run a heated bed), lcd with voltage/amp readout (upside down the way I've mounted though), already has an IEC connector built in + no need to mod like PC power supply. Fit was too tight for inside the hatch so decided to mount externally with zip ties.
*120mm fan and filter from an old HTPC case - just cut a hole with a jigsaw and drilled 4 holes for bolts on the back hatch panel, then have the fan powered from a screw terminal behind the hatch. Printed fan spacer (red) because the fan blades were grazing the filter.
*2 x ikea lots mirrors (pack of 4 @ AU$14) - needed to raise bed height a little because I was having endless troubles trying to get the z axis not to bind towards the top - short of a major rebuild I've tried most suggestions.
*Not sure if visible in the pic, but stronger/thicker bed springs + some longer M3 bolts x 4 handily left over from the kit.. The springs are also squashed between some thick m6 or m7 nuts (forget exact size) to add a little more bed height to clear the axis binding better and pre-load the springs tension a bit more. Finding I'm barely having to readjust the level my bed so not going to bother with installing the BLtouch. Will probably remove said nuts and go back to springs only once I switch to e3dv6 + titan.
*No heated bed power connected - I run Glue on glass, PLA only. My melamine bed piece was already warped from the month it took shipping, never mind regularly adding heat cycles to the equation. If I do eventually add one it'll be silicone heat pad + cork + foil tape + 2020 bracing underneath the melamine (similar to google photos linked above)
Cork stepper dampers for the x/y axis are the only other mod since these photos, cheap, and definitely took some of whineyness out of the y axis.
Tried printing the low profile scorpion(?) part cooling fan from thingiverse and it wouldn't clear my nozzle. Not sure if I can be bothered adjusting my heat break to compensate, probably rather spend the time installing the e3dv6, but currently in no rush and want to stay stock a little longer. Tested with a turbo fan, far too noisy so for the time being I'll try another part cooler design with the spare 40mm fan I didn't use for cooling the board.
*flange bearing/pulley mod sounds like an inevitability
*possibly z stop mod - although mine works fine it does seem like a weak point.
*MDF cabinet on wheels - printer currently resides at my office, but will shortly be taken home.. need an enclosure to keep gf's cats away, not to mention help with the noise.