I've built https://uptimelog.app as a small side project to scratch my own itch. It's free for now, so I don't have much feedback on how to build a business around it yet. However, I do think MacOS development is definitely a niche in itself, which can be a good thing because you have a user base that needs software for a specific platform.
I initially used Go for the backend and web as the frontend, so it essentially works as a local web server serving a web page in the browser. However, I'm now rewriting it in Qt (C++) to make it easier to maintain and to make it into a native application instead of a local web app.
For example I've tried this one: http://gis.stackexchange.com/questions/102555/automatically-.... Only question text saved into document.
As for the book from urls, there could be some options how deep to scan urls, only from current page or all subpage, etc.
Nice UI, but it doesn't work in my case. I've tried a bunch of urls from stackexchange and the output is not readable (not properly formatted). But anyway, what I suggest to do is to add feature that I'm looking for (and maybe not only me?) is to convert website to some kind of book, for example in pdf. As I remember Adobe PDF has this functionality included, but I 'm not aware of any web app that can do this. So, basically you can parse whole website and create book out of it. So, anyway good luck!
I initially used Go for the backend and web as the frontend, so it essentially works as a local web server serving a web page in the browser. However, I'm now rewriting it in Qt (C++) to make it easier to maintain and to make it into a native application instead of a local web app.