Whenever I have started a serious side project it changed my life in 18 months or so. It goes from "not getting called back" to recruiters (not from Amazon) contacting me.
Usually my side projects involve finding some use of a new technology that other people aren't doing that doesn't seem that hard. Some examples are
* In the late 1990s I made a Java applet that knocked off Apple's Quicktime VR
* I built a web site that had an XML-based back end the week XML 1.0 was finalized and got a deal to write a chapter in a book
* I made a web site of animal pictures, went on to make sites about cars and New York City, then made a list of topics from DBpedia and attached over a million images with near perfect accuracy
* I made a workflow system for handling job applications and classifying job listings with machine learning that got me the first job I applied for
Right now I have three side projects that form a series
* a system for making "three-sided cards" that have a photo or art reproduction on the front, a description and QR code on the back, an associated web site as well as installations of the cards that have "digital twins" like
https://gen5.info/$/XQ*42RXF-TLY:$B.8/
that one is nearly wrapped up.
* a persistence-of-vision display that mounts on the passenger side of my car (I need to have it perfectly debugged before I drive with it so I invented an error-correcting communication protocol to send graphical data back to my computer)
* I want to do a sketch comedy act with a video game character projected like
https://en.wikipedia.org/wiki/Pepper%27s_ghost
which is a "moon shot" for me because it involves software, hardware, being funny, finding a venue, etc.
If I were looking for a new web-based project I'd use WebGL to give people a videogame-like experience. You can't make an AAA game but maybe you can make something like the trailer for an AAA game. In general I am seeing videogame interfaces (say the Pokédex or other "guides" you find in a game) as an inspiration for mobile interfaces.