With one-third of the current Star Wars trilogy left for J.J. Abrams to come back to, we could very well learn about Snoke’s other apprentice in 2019, and learn that it’s someone else out there in the galaxy.
Building off of this development, we will have to wait and see if it is expanded on in Star Wars: Episode IX.
The possibilities are seemingly endless, ranging from Rey (perhaps she just can’t remember?) to Emperor Palpatine to Dark Plagueis.
As mentioned earlier, Supreme Leader Snoke left the world of Star Wars as abruptly as he ended, which means much of his backstory remains on the table and ready for exploration.