Future archeologists are in luck, this already exists: http://www.emojidick.com/

That's the Unicode specification, isn't it? (Though I suppose that doesn't get the in-prose meanings.)

Why though? All the emoji that people actually care about depicts faces or objects with obvious body-part analogues

ancient hieroglyphics also depict common objects and expressions. their combinations carry even more meaning. the understanding of an eggplant, a thumbs up, or even a smile could be drastically different in 5000 years

No because we have a recorded archive of this stuff. Ie: Wikipedia.