Here are some features I would like to be added:
1. That you can see chests and other things with a GUI. When I was playing on Mineplex, I got out my Wither Morph, and when I viewed the replay, I could not see the Cosmetic Menu, all I could see was me holding a chest. Since most servers use GUI Chests like that for game menus etc., that is an important feature and should be added. I think that this should only be shown if you are spectating yourself.
2. Hotbar. I understand why you cannot see other players' hotbars' while spectating them, but I believe that it is possible to show your own hotbar (correct me if I am wrong) when you are spectating yourself.
I think these features are VERY important if the Replay Mod is going to replace screen recorders like Fraps.
Howdy, I'll need my fellow beta testers and mods the help me out on this but to my knowledge as the mod only captures what is sent to the client from the server those Mineplex/Hypixel/etc style gui may not be transmitted in a form for the mod to interpret it. Please remember the mod is to work in a vanilla server environment first and for most. So any custom 3rd party custom things sent by the server may not be able to be interpreted.
Hotbar may be possible however from the point of view of the spectator I believe it doesn't distinguish one play from the other. This may be a minecraft issue and not missing feature but I'll need CP to confirm.
As for replacing Fraps and I can say with a high level of confidence I don't believe that is what CP intends as I've personally seen him say to take clips from the mod and do final editing in Premier or like program, which I agree with. If I may suggest recording your POV and matching it to your 3rd person clips (example: https://www.youtube.com/watch?v=Omn3k61JirU via SlicedLime)
It was never intended for this mod to replace screen recorders. The purpose of the mod is more to allow better cinematics - timelapses, slow/fast motion replays (like the name), and anything that would normally require having a second account recording from spectator mode.
I believe server-specific resource packs are either planned or already coded in, but inventories are highly unlikely; especially because the mod can only see information sent by the server directly to the player who is "recording" the replay.
Even if it was not intended, it would really be very useful to be able to use this for capturing gameplay as is without the need of an external screen recorder because of the ability of rendering without the need of being in time. You can constantly claim the FPS without the worry of sudden frame drops. At least it would not replace screen capture software as it can't record your screen, webcam etc. but only Minecraft in-game gameplay.
EDIT: PS: It would at least enable the recording of 360 degree Lets Plays, which would create a totally new experience of interactivity.