I'd like to discover niches where people are stuck using software or services that they hate, but where there aren't viable alternatives.
What are you forced to use but absolutely hate using? If there are viable alternatives, what barriers are there to switching?
I'm less interested in hearing generic "I hate Microsoft, but have to use it for work" statements.
I'm more interested in hearing "I work in [industry] and have to use [bad software]. It sucks because [reasons]. We haven't switched to [alternative] because of [more reasons]."
When I worked in scientific computing, one of the founders of the company I worked for had done doctoral work in the areas the software was covering, and the company hired domain experts, who also happened to know a bit about software development, whenever they moved into new fields.
This is the reason I've never gone into medical billing, despite the fact that I know the medical billing software industry is a mess, and also an incredibly lucrative market. I know it is a mess, because my sister owns a medical billing firm, and I've seen the software her company uses. But, without spending a few months working in the field, I wouldn't be effective at solving those problems. If I ever did decide to start a company in that field, I think I'd need to spend some time in the trenches of that particular field.
Likewise, I suspect if you do find yourself a lucrative and currently poorly served market niche to focus on, you will need to also find a few companies to work closely with, so you actually understand the problems and how they can be solved. Also, keep in mind that the harder existing software is to use, the more training existing users will have had to know how to use it. Don't underestimate how hard it is to convert someone from using "complicated but familiar" solutions to using "easy but new" solutions.
Not seeking to rain on the "I hate X" parade, just pointing out that if you aren't scratching your own itch, to some degree, you have a lot of work to do just to begin to understand the problem domain.