Since I decided awhile ago spoiling this was NOT going to be detrimental to my desire to venture into SWTOR, I'll indulge here despite not being able to play yet or for a foreseeable time.
He disappeared in a blinding flash of sparks and light. One could speculate
a> that was the end of him and he became one with the force
b> he is no longer with us on a physical level but who knows if he isn't hanging around on a force-apparition level?
c> he transformed and became something else.
For the last one of course I have one idea: is it not possible he resided within his own holocron? Of course the device would probably work regardless. If you are familiar with Drew Karpyshyn's earlier work, you will know what became of the holocron of Revan in the book Darth Bane: Path of Destruction.
The device worked and taught Bane everything it knew, but due to an impurity in the crystal used in the device's construction, the device degraded with each time Bane used it. At the end of the final lesson, after the last glitch just before it died, the holocron of Revan said to Bane: "My time here is ended. Take what I have taught you and use it well." And then Revan was gone. The glow emanating from the holocron faded away to nothing. Bane retrieved the small crystal pyramid from the floor, but it was cold and lifeless in his hands. He felt no trace of the Force inside it.
The artifact was of no more use to him. As Revan had taught him, it must therefore be discarded. He let it drop to the floor. Then, very slowly and deliberately, he crushed it with the power of the Force until only dust remained.
Unless there was another lightsaber or lightsaber crystal that once belonged to Revan, I'm afraid there is little else left in terms of physical traces of his existence.
Ancestors such as Deena Shan in the time of Luke Skywalker.
HK-47 lived a looooong time afterwards, too. After the clone wars, a crew of pirates or mercenaries attempted to capture and/or destroy HK-47 in his mustafar facility. For their efforts they got a mockery gift. A hologram of HK-47 with "meatbag" engraved into the base of the projector. So as of the original trilogy, his status is at large and unknown. He did not reappear 135+ years later in the Legacy series.