This server-side version of the Replay Mod would record everything that happens on the server, much like the normal ReplayMod. When the server is closed, you can download the Replay file and use the normal Replay Viewer to watch it.
This would be useful for people who want to record on a private server. It would solve a lot of the annoying limitations of the client-side ReplayMod, because the server knows everything that's going on. It records all the players, not just the area around yourself. It also records things like the contents of containers or player inventories, which is something that can't be done on the client-side version. I think this would be a great way to increase the quality of many Replay videos.
Lastly, this would be very useful for griefing prevention, because you can see everything that everyone has done.
I know this would probably be a lot of work to create, but it's just an idea.