So many YouTubers have already done videos on this subject and to be honest, I haven't watched them, this is just from a conversation I had with Aly (@Awesome_Sweet)

So because in Rebels they will be going to Malachor it's possible that Revan could appear, now the last KNOWN sighting or word from Revan is when he disappeared into the Unknown Regions so it is POSSIBLE he stayed there for thousands of years using his immense power in the force to keep him alive and has recently (since the fall of the republic and rise of the empire) come out into the known galaxy to, destroy or seal ancient Sith temples.

Now the Sith Emperor/Valkorion hid their to rebuild the Sith empire but that was only for 20 years (correct me if I'm wrong) so it's not likely that Revan is still alive but hey this is Star Wars, and well Revan is a fan favourite so anything could happen.

Anyway I think that he won't appear in Rebels, cause there's too much to explain regarding how he got there so on so forth.

Well that was the first of my theories to crack/craft and I haven't decided what the next one will be yet, I'll talk some more with Aly so see Ya later guys and girls