MineCon capes are always about recent updates. 2016 was an enderman, because the 1.12 update was..end based.
There are not many informations about the upcoming 1.13, but we know about new 'customisable crafting recipes' - so maybe a crafting table?
Mojang is planning to change some textures in the new update (by Jasper Boerstra), so it can be anything. And, maybe new mobs will be added?