There is always that moment when you make a replay, and you are almost out of frames. It would be nice to have way to add frames to a replay. And or remove them.

You can do this manually.

Not really. You need to open the replay with notepad and edit the frames once you've saved it. Probably won't work. But you can edit the Replay with more frames after notepadding it and there you go. Or just analyze your replay with a script and do it again with more frames.

It would be easier just to have it as a feature in-game. Some people won't know how to do that.

I have another suggestion as well. You know the feeling when you have some awesomeeee moves in your replay, but you want to improve one small detail in it. But that detail is in the middle of the replay, so that if you change it, you gotta have hard time repleating all the further moves.
Implement an option of an Alternative Editing Mode of Toribash replays, where you can make changes, which rewrite the move of a specific joint at a specific frame in the time, and the rest of the later moves continue as they should. This can make perfecting replays so much better and so much more fun.

Ideas?

You can do this manually, Just have to have some knowledge of how TB's replay save system works or at least remember what the amount of frames there is, maybe ill make a tut. on it.

You have to remember that the rest of the replay will be changed.