I have built various stuffs in the past few years and have came to appreciate more and more on things that will never get tired of. It can be a good indication that the problem spaces are large and remain unsolved as well as the opportunities. In such case, there are always rooms for anyone to come in to innovate and put their own spin to it. This is sometimes better than working on something that few or barely anyone works on, which can be an indication that the problem space is too small and only affects small set of folks.

Love the list btw. What is an example of "personal knowledge base" type app? Like Notion?