Another thing that I always get a PCB back and smack myself for not including: adequate ground test hooks. For low speed stuff, having a nice ground loop to easy-hook your scope to is really convenient.
I also tend to kick myself when I don’t include color coded test hooks. I bring up a multicore ARM SoC once or twice a year, and these make it really fast to validate a power sequence. These hooks make power/reset sequencing a breeze. You can just clip your scope probes to the next hook, cycle power, screencap the sequence, and keep moving. Be sure to include a hook for the global RESET_L line as well - that is frequently a useful timing reference for the overall sequence!
It’s an extra ten minutes to add to the PCB, and an extra $100 of test hooks (probably less if you get them from the Chinese equivalent of Keystone), but when you’re dealing with a multicore ARM SoC with an abundance of voltage rails (looking at you here, Mediatek 
), it can turn a half-day or day’s worth of measurements/screencaps into a half-hour.