This idea is mainly for sparring, but can apply to duels and other things. It's really a shame to have a great spar going on, then someone has to leave with 500 frames left, leaving the game unfinished. One option is for one person to edit the saved replay later, but the second person won't have any say in what his character does.

My idea is that in multiplayer, the op would be able to load a replay and set the exact frame at which the game will start, just like you can in singleplayer by pressing e. This way, any unfinished games (as long as the replay was saved) can be easily finished at any time.

For example, I think a command like
"/load mysparwithpindar.rpl 1560" would be sufficient. That would start the game at frame 1560, right where we left off.

This idea could be used in other ways. For example, you want to start a game in a position other than the default position that tori and uke always start at. You could go into single player and move uke and tori however you want, save the replay, make a note of the frame you stopped at, then load that replay to that frame in multiplayer and just like that, you can start multiplayer games in any position easily.