Technical analysis of stock prices etc. I have the mathematical background to do it, but I only did one semester of programming (Java). Enough to know that I can code. There seems to be a lot of software which does this sort of thing already, some integrated trading platforms, some programs which take user supplied data and churn out analysis. Mostly sold by monthly subscriptions at varying prices but there's a lot of added value over raw data. Maybe a crowded market but there doesn't seem to be a single ubiquituous piece of software used by everyone as standard. Given time I think I could replicate most common features and maybe come up with something novel.
The scope of the work that would be needed seems greater the more I think about it, I think I would be best off working on the number crunching first and worrying about making a marketable user-friendly interface later.