Both your suggestions seem very useful.
Possible rather then optiboot specifically, a general bootloader sticky would be useful with embedded links to specific bootloader tasks one might be looking for. The problem as it stands today is that there is so much legacy information and past solutions that now have better solutions. It’s probably a much larger subject then the loop-back task, but ultimately very useful.
Millis() also could use a good one stop home to explain the why and how of proper usage. What really would be helpful in my opinion is a better example then the blink without delay sketch, one that would show two different timed events being processed independently from each other in the single sketch.
I would think a basic topic of proper wiring of simple LEDs and why a series resistor is required and how to calculate it’s size.
Another might be how to wire up simple switches using proper pull-up (internal or external) or pull-down resistors, and the why and how to handle switch contact bounce in sketches.
It’s not that these topics have not been covered in so many past posts and tutorials, but rather because of the vast spread out nature of the information, new stickies give us a chance to condense, simplify, and update the needed information so that we have a better common reference source to site for newcomers.
I’m more interested in the quality of the stickies created rather then a rush to see how many can be developed in a short time span. As we saw with the loop-back topic, it’s takes time and thought to find the the bare minimum number of steps and words that cover the topic accurately without excessive, redundant, and possibly confusing or conflicting information.