Star Wars: The Last Jedi Trailer - 13 Easter Eggs & Secrets You Probably Missed

2. Force Tree

Going back to the books we see on Ahch-To in the trailer, one of which is probably the Journal of the Whills, and we can see that it looks like they're sitting on and surrounded by tree branches.

A Force-sensitive tree once grew at the heart of the Jedi Temple on Coruscant, which served as a school and monastery for the Jedi Order. It was removed after Darth Sidious assumed control of the galaxy, and just two living twigs remained, hidden on an old Imperial base. They were retrieved by Luke Skywalker and Shara Bey (mother of Poe Dameron) in the comic Shattered Empire IV. Luke gave one to Bey, who planted it outside her home on Yavin IV, but kept the other for himself.

All that is canon, so there's a good chance that Luke planted that twig on the planet where he established his new Jedi Academy, and it appears he took part of it with him when he relocated to Ahch-To in search of the First Jedi Temple, because it looks like we're seeing a Force-tree here.

Also, if you brighten the image of Luke at the end, you can see that the cave he's in isn't a cave at all, but the inside of the tree (and you can make out what appears to be Rey's staff as well). The Force tree was seen as a metaphor for rebuilding the galaxy after Return of the Jedi; it'd be quite symbolic, then, if it was to be destroyed here, as the Jedi may come to an end, and the First Order at looking to take control of the galaxy after the destruction of the Republic.