Thought I'd make a list of things that point to Ranboo perhaps being the last time traveler. If you have any more, please comment.
Slime. Slime didn't know who Ranboo was when they first met. Which was really weird.
Lessons. Ranboo spontaneously remembered lessons during lore awhile ago. We all thought Dream taught him them, but what if it was the Overseers? They brought up lessons to Karl.
Black and white. Ranboo has a black an white theme that can only be half explained. But Karl's hoodie was black and white in DreamXD's world.
Amnesia. Ranboo frequently forgets things and remembers nothing from before he joined. The last time traveler forgot they were a time traveler.
Enderwalk. No one knows why he does this, it could maybe be a side effect if his time traveling.
Diaries. Both Karl and Ranboo keep journals to record their memories.
Relations to Dream. Ranboo seems to occasionally do Dreams bidding. If he was the time traveler, it might make sense that he'd gravitate towards the person most like DreamXD. Especially if the Enderwalk is due to his time traveling.
Lives. We don't know when Ranboo lost his lives. If he was a time traveler, he could have lost them on the job.
People not sides. The Overseers want Karl to pick a side, desperately. If Ranboo was the last one, he'd probably gotten the same treatment, and might be salty about it.
