Unicode Technical Report #51, which is where Emoji are laid out, talks a bit about the current thinking of the committees on this:
> The longer-term goal for implementations should be to support embedded graphics, in addition to the emoji characters. Embedded graphics allow arbitrary emoji symbols, and are not dependent on additional Unicode encoding. Some examples of this are found in Skype and LINE—see the emoji press page for more examples.
> However, to be as effective and simple to use as emoji characters, a full solution requires significant infrastructure changes to allow simple, reliable input and transport of images (stickers) in texting, chat, mobile phones, email programs, virtual and mobile keyboards, and so on. (Even so, such images will never interchange in environments that only support plain text, such as email addresses.) Until that time, many implementations will need to use Unicode emoji instead
