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).

The module needs these changes to work with Azeroth Core: https://github.com/laasker/AzerothCore-Arena-Replay/commit/b4218bf4d8458299dc65b4f82b713b8aa46bf6af

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.