Best purchase as mentioned above is the 7500 planarite focus. One for each "role" you fulfill, IE tanking, dps, healing, support, etc, if necessary. If all you do is dps, than likely one will be fine. But the same one won't be good for both tanking and dps, for example.
That's 15k. Then you will want a ton of Ancient Sourceshards from Stillmoor/Shimmersand/IronPine invasions/events. Fill those focuses with the essences that are 800 planarite/3 Ancient Sourceshards each.
15k for the medic is very handy, especially if doing something like Expert Iron Tombs as a Guardian character - no healer anywhere near so a pocket healer is nice.
Vanity pets are a thought, sure.
What I've found to spend extra on after all the above though is the Master Augment Box in Sanctum (and guessing Meridian) for 500 planarite. Contains the rare crafting augments. Some sell quite well on the ah, or if you are a crafter they're useful for you like that.