Fix all content up to 90 first. New players play through 1 to 90. 1 to 90 is horrible. Its fast and smooth if you know how to level, what zones to avoid and in general understand MOP (which is why people who already are past it often think its fine, because they now can easily do it). But if you dont, it sucks. First character took 20 hours played from 1-90, second one was done in 7 (and i played Mop before and leveled multiple characters).
Afterwards, all of Pandaria needs to be fixed. Its the current expansion, at least it should work flawlessly. But (besides cata) its in a bad state.
I really wouldnt try to win players over before that has happened. If you allow players to transfer (i.e copy) characters over from woltk now theyll see the state of the server and leave and they wont come back. The server will get a bump now, some players might even stick but in the end it will go downwards again. If the server ever is in a A+ status (like woltk) -> then its time for advertisement and transfers and free boosts or what not.