Four pillars: passion, authority, audience, profit
how-toFour columns, four to eight candidate topics, one to ten in each. Add them up.
Passion comes first and is tested by whether you could talk about it stuck in an elevator. Authority is an honest self-score from 'not accomplished at all' to 'like an Olympic athlete'. Audience is researched rather than assumed - how many channels exist, how many views, how recent, and whether Google Trends shows interest rising or falling. Profit is checked by looking at what the top creators in the niche actually link in their descriptions, and by Amazon review counts as a proxy for people already buying. The winner is the topic with the highest total, not the highest single column.
A defensible niche decision in an afternoon, instead of six months of drifting.