If you've ever used LINE, they have a really nice inline emoji suggestion function, where as you type words a list of possibly related emoji appear (presumably they have them tagged with various keywords).
Everything else about LINE is terrible, but I'd love it if everyone just copied that particular interaction.
Swype does this too, although I never really use them, it is a cool feature.
Swiftkey has this, and it really changed the game for me because it baked emoji discovery directly into my typing flow.