Master Yoda
While Obi-Wan watched over a young Luke Skywalker on Tatooine, his former master Obi-Wan isolated himself on the Dagobah system following the events of Revenge of the Sith.
While on different planets, there would still be ways for those who are strong in the force to communicate with each other, and both Yoda and Obi-Wan are two of the most powerful force users known in the Star Wars Universe.
We know that there is precedent for Yoda at the very least to communicate with force users from another planet, as we saw in Star Wars: Rebels, when he spoke with Ezra while he meditated despite being on Dagobah.
Yoda could appear and give guidance to Obi-Wan as he struggles to adjust to his life in isolation, or he could also appear in flashback to Obi-Wan in his younger days before the fall of Jedi Order.
Regardless, Yoda is one of the most recognizable character from all eras of Star Wars, and given his connection to Obi-Wan, it would make sense for him to have a cameo at the very least.