Anyone can submit ideas for new emojis
In all likelihood, the idea to add an avocado came from someone like you or me. If you're desperate to get a new emoji added to your arsenal, you're welcome to submit a proposal to the Consortium as a civilian. However, in order for a petition to even be considered, it must adhere to a strict and incredibly specific rubric created by the gatekeepers that addresses 10 different selection factors. How frequently will it be used? How necessary is it? Is there a measurable demand for it?
You can't just BS your way through it, either -- the Consortium demands real evidence that people want your emoji. Of course, you'll also need to submit prototypical imagery matching your proposed emoji, and it must adhere to a specific dimensional format and style. Every emoji on your keyboard, and the avocado, had to at one time pass muster.
You don't necessarily have to do all the work yourself, though. The site Emoji Request lets anyone vote on potential new emojis, then mercifully submits formal proposals to the Unicode Consortium for the winners.