Seats !!!
Now those are important items. When you sit in one for hours and hours, day and night, they need to be comfortable, with good adjustment, in and out, up and down, thigh support, lumbar support, and all.. Recall one of the best I have used was in an ATR, and the prize for the bad one was split between Airvan (not enough adjustment, back too straight) and the bare Nomad mesh one. Even my Cessna one seems to have me too close to the side of the fuse for long trips. If you are building your own aircraft, then it should be a priority to get the ergonomics right for enjoyment later.