I've been searching for a good while now. The fic is at least 4 years old I believe. It's a sadfic about Twilight becoming princess, and in the far future, she's discussing the loss of her friends with a pupil of hers. At the end, it makes a metaphor implying that a circle of 5 stars in the sky represent her friends, and there's a spot open for her.

EDIT: the title of it popped into my head today. https://docs.google.com/document/pub?id=1zZZnizNM_2hh_UQ22gmk2VChBrlFCmyLevNuCbKbSaQ