They never said that they will never add a new rank again. There simply isn’t need for another partner/staff/other rank at the moment. If there was a new social media platform blowinp up for example, they would probably consider adding a new partner rank for it. 
But to get back to the main suggestion, The Hive will likely not add a Twitter rank, as it doesn’t have much to do with content creation. Sure, you could upload videos to Twitter, but it is mainly a discussion platform, not for content creation as already mentioned. :)
Also this is a duplicate suggestion, feel free to vote for the original thread that suggests a Twitter rank here: