I feel like this finale could go one of three ways:
Theory 1- Donna dies. She displays some next-level heroism, and gets herself killed. The Titans mourn, but it’s okay because she’s with Garth now.
Theory 2-Rachel dies. If memory serves, her little card reading session mentioned something about sacrifice, so it’s possible that she throws herself into the line of fire or whatever else and winds up dead. Titans mourn, but because of her link to Trigon she ends up coming back to life at a later date (perhaps a cliffhanger for Season 3?).
Theory 3- Jason dies. After he makes his way to the fight, he gets killed and is remembered by the rest of the gang as being a Titan. Bruce Wayne comes to the funeral for one last appearance in the season, where he is frustrated that Dick could have let this happen, but ultimately punishes himself for everything.
All three of these characters have died in the comics at least once, so it will be interesting what the writers of the show decide to do.