We'll still need some sort of 8.3.5 or 8.4 for an end game zone to keep us busy for the next year until 9.0. 8.3 isn't going to carry us the rest of the way.
Most of the island expeditions pointed towards future content. Even stuff which seemed random at the time - like Tol'vir - eventually pointed to Uldum in hindsight. We now know looking back that Black Dragons made sense. The things we still haven't dealt with too much have been all of the Pandaria mobs (mantid, mogu, etc).
If the Yak/Monkey are true...
Hiji seems likely.
Alliance will probably get the
Lightforged Undead like Derek Proudmoore. OG Forsaken might get an upright version.
Horde will probably get the
Troll Saberon shapeshifters from ZG/ZA - they'll use the new Worgen model and animations. (
See the mobs in this WQ - Halazzi and the boys)
Mogu are a potential allied race.
Jinyu just got updated with the Ankoan.
Lots of hints point towards Pandaria.
Timeless Isle was probably one of the better end game zones ever made. Makes sense they'd push a Pandaria island.