[ken]

Now you're getting into the definition of "writing". I would say I've only seen emoji typed, not written. (Before anyone asks: yes, I've seen cuneiform written. I have some interesting friends.) If you count any visual communication that is typed on a phone under the greater umbrella of "writing", then we could also include colors, styles, orientation, funny fonts, image memes, animation, etc. There's no end to the possible visual communication that people might want to transmit digitally.

Where do you draw the line? I draw it at "anything in or using a language that people might write in the absence of computers, which they would then reasonably want to store and transmit using a computer". I don't include "any possible visual communication that can occur using a computer". That's far too broad to define "text", or be part of any existing "language", which are the stated goals of Unicode.