The PTR is slowly getting updated with upcoming 7.3.5 content. Here's a list of new discoveries/changes.
Build 25600:
New pet - Mailemental (Magic), has fun theme-named abilities such as Snail Mail, Return to Sender, Hate Mail. Likely inspired by an idea posted on the Bnet Pet Battle forums! Obtaining this pet may require completing a quest chain.
New Pet PVP achievements - Similar to Family Familiar/Fighter, but for PVP. Meta rewards a tabard. (thanks for the heads up Jodmos!)
Updated master post with additional notes on the possible way we'll collect Mailemental, plus some new details on Uuna (spoiler warning, read the dialogue in the Wowhead link at your own discretion).
Feel free to browse through Perks N Peeves, my vanity pet collecting blog!
What exaclty is supposed to be in next PTR? isnt this all we get in legion? Besides the class changes, raid, and miscellany in 7.3.5, Ion in the last Q&A said there will be one or two more minor story patches forthcoming as well. So I'm expecting at least a 7.4.