What are good quality of life additions to a prototype PCB?
I'd add large silkscreen rectangle, pref.on both sides - to add various Sharpie marks when necessary. Lots of small bare copper islands to store SMT parts you have just removed but may need later - just solder one pin to the closest island :-). Extra non-tented ground vias next to likely probe points - to solder a ground wire that will then be wrapped around a probe. Extra 0.1" jumpers where you may want to clip your current probe.