I think the only formula that will not grow stale is one's own authentic voice. And I think it does need to be applied to an ever-changing range of subjects to remain fresh.

Niche in Spanish - nicho - seldom refers to a specific subgenre, but rather the cavities in cemeteries where coffins are placed to decompose.

And the word apparently came into English from an old French term for a dog kennel.

I think free, live humans are better off steering well clear of niches. They are full of musty, crumbling, gnawed bones.