Patch 5.4 has plenty of new features to offer, but at the present time many of those features haven't quite been released yet on the PTR, including the Timeless Isle. What's the Timeless Isle? It seems to be home to the new pet battle tournament that was discovered, as well as some sort of temporary hiding place or home for the August Celestials as well.
You can't access the Timeless Isle on the PTR currently, but Adriacraft has managed to put together some footage of the Isle in detail. The video is after the break.
Please note: This post contains possible spoilers for patch 5.4.
At the current time, the Timeless Isle seems to be home not only to the pet battle tournament, but the continuation of Wrathion's quest chain. Datamined quest files suggest that players will be sent to the Timeless Isle to collect 50 Timeless Coins on the next leg of the journey. Exactly how quickly these coins are collected is unknown -- the quest suggests gathering them from creatures and looking for treasure hoards -- but Wrathion does shed some light on the Timeless Isle. Apparently it's some sort of anomaly that appears and disappears at will, an odd island where time has no meaning.
What does this mean for players? Well, judging by the video above, it looks as though this may be another Isle of Giants, although with much more of a purpose. It's out of the way, seems to include flight paths for both Alliance and Horde, and contains a mess of critters and some world bosses in the form of the Celestials and one very angry yaungol. We'll have more information on this mysterious location and its purpose once it's released on the PTR -- until then, enjoy the video from Adriacraft and check out the gorgeous scenery.