So right now on the 5.4 PTR, attempting to enter the Timeless Isle immediately causes you to be teleported away, as the zone is still in very early development. However, if you're very cautious, it's possible to swim in the water juuuust outside the Fatigue belt and explore the island with Eye of Kilrogg.
I present: An album of my finds.
Highlights: Alliance and Horde have their own distinct landings / flights points.
Each of the four celestials present as World bosses (!). Seems to be four active at once...? Maybe all in one as some sort of superboss encounter (a la Oondasta)
Skunked Alementals (dailies?).
Pulsing Wisps (?). Very similar to that star trash in Vortex Pinnacle.
A fifth world boss with much more health: Ordo <Fire God of the Yaungol>
A building with a very striking similarity to the temple of Nizuao and a ruined bridge.
<Personal theories below, please take 0 stock>
Personal theory? The original temple of Nizuao, lost at some point (or perhaps simply built to contain Yaungol). The Celestials being aggressive leads me to believe that they have been driven to hatred / hostility towards the Alliance/Horde for their actions (the destruction of Vale, as they are the guardians of the Vale). Perhaps... And I fucking dread this word... Corrupted.
<Personal theories ended>
Island is unpopulated except for the mobs I listed (I may have missed of course)-- and for anyone curious and who notices the cave near the Celestials, I tried very hard to reach it but the Eye just doesn't move fast enough.
Will try to snag more if the servers stop being so ridiculously unstable.
Edit: Second album incoming due to a gloriously lucky bug with the servers being so unstable. Album will only contain images of terrain.
Edit #2: Part 2, exploring with with a client-side bug.