For me it's really more about the plate material, switches, tuning of switches & stabs, & sound dampening steps taken that make the ultimate board. Sure it is very nice to put all of my favorites of those into a high end custom case, but at the end of the day I honestly think it's the time & care taken when putting a board together that makes it special or not. I mean think about it if you had the choice of using a say a Keycult No1, but it has pre re-tooling MX browns that are unlubed, knock off clip in PCB mount stabs unlubed, & the plate is a material you do not like. Or the other option would be say a KBDfans 8X that has properly lubed & filmed Holy Pandas, GMK screw in stabs clipped/properly lubed, the plate is made from your favorite material, & proper sound dampening has been done between the plate, PCB, & case. Which would you choose to use? Personally I'd go with the KBDfans 8X as it will feel many times better than the Keycult No1 in this scenario. So honestly I would put much more emphasis on the little details to get my ultimate MKB & just make sure I select a decently high end case that is made of alum. or PC that I really like the design of & comes with a high quality PCB.
In fact I'm in the process of that right now TBH. I recently got my E6.5 & went for an aluminum one with a brass plate. I plan on using MD HPs lubed with 3203 on the sliders/housings, TX's synthetic paraffin oil on the springs, 70g TX springs, & white switch films for the switches. GMK screw in PCB mount stabs clipped & lubed with Christolube MCG129, then use MK ultra 65% plate & case foams for sound dampening. That is honestly my ideal ultimate MX based board. I guess if we want to get into wishful thinking territory if the E6.5 supported ALPS SKCM/L switches & I could put the batch of damn near NOS SKCM browns I got in it, then it would become my ultimate MKB no questions asked!