Eng - He had hit upon a potentially lucrative niche ?

I think should be like this.....

niche, 本義簡作牆壁上的小閣

Niche: A shallow ornamental recess or hollow set into a wall, usually for the purpose of containing a statue or other decorative object.

但似乎在這情況用以下的解釋較好
Niche:Business (orig. U.S.). A position from which an entrepreneur seeks to exploit a shortcoming or an opportunity in an economy, market, etc.; (hence) a specialized market for a product or service.

則解作：他已遇到（擊中）一個潛在有利可圖的機會（點／平台）

註：（）內是直譯單字意思

Hope it helps