I've been keeping a page in notion for a few years now and it has hundreds of ideas. Lately, one of my bigger sources of app/software ideas has been building dinky little side projects and thinking of ways to improve or automate or make my life easier.
The two most important aspects of this (for me) are: all ideas get written down immediately; and I do not make any judgement about the quality of the ideas/thoughts that get written down. At least, not in the moment. Assessing the value of the ideas comes later, when I'm mining through the notebook.
Some people suggest launching the marketing / sales first. Fraud then probably. But having more than yourself as a feature guide right from the start sounds more like it: either existing competition or some potential clients at least willing to talk to you - which also helps pricing effectively.