Confirmed this week by Jasper Boestra, aka Jappa, no new textures will be hitting the game in Minecraft 1.13:
1.13 is a technical update, check out @Dinnerbone for more info about that! My textures wont be included.
Jappa joined the Mojang team in 2017 and recently has been reworking the textures in the game, with the idea of having a unified feel throughout. The current textures are a mish mash of development eras and contributed to by various members of Mojang over the years. Fans had hoped to see the textures arrive in 1.13 but this tweet finally dispels the chance.