The arena replay module allows you to watch replay of rated arena games (I think it can be modified to Replay BGs and Raid instances aswell).
You can see a little bit of how the module works here: https://www.youtube.com/watch?v=7z0RA6Dsm9s
it's not 100% done yet because if a player tries to watch a Replay of a game that he was in, the replay acts weirdly. Words of the guy that updated the module: "try spectating as a player who wasn't involved in the arena I need to change it where it uses new duplicate players instead of"
Credits goes to @Romain-P who is the guy that created and shared the script https://gist.github.com/Romain-P/069749c3acced35f4b0ae6841cb94e79 and @thomasjteachey who updated the code to work with recent Azeroth Core and also did the Core Changes needed for the module. I only did the npc gossips options and added a few missing opcodes.