Ask HN: Which is the richest IT niche for a 2-3 years project?

I’ve just been asked this by a couple of young engineers in search of a new project for quick and good (€10+m) monetisation in, say, 2-3 years and starting now from scratch. I have put fintech cybersecurity forward as the most liquid and possibly the richest niche out there for a 3 years attempt. Any better tip? Thanks. EDIT: project starting from the EU.