Some major brainwaves just came to me when watching some Rocket League edits. How about the ability to use the replays as simulations for training purposes (with or without all players present)? I mean the best way to learn new stuff is to be able to learn from mistakes. But the training packs now are just shots that make you exercise your skill and never truly reflect your shortcomings.
The replay save files must save all mechanical values in a game, otherwise it can't simulate the replay you're looking at from all perceivable angles. So if the replay viewer would show a feature to the likes of "feed current snapshot into a 10 second training", it would feed all the current mechanical values (of the ball and your car) in a training in which you can re-apply yourself and see if your mistake was fixable or just redo that shot until you get it. The notion of also including the players is interesting; it could just simulate the players with all the inputs made in these 10 seconds, but messing with the players' cars would yield some funny results. So this leaves some question marks, but it would yield a very practical training.
It could be that I'm entirely mistaken and that the data from a replay is not sufficient or compressed in such a way it can't be realized in an active environment anymore, but I wanted to at least share this idea and see what others think of it. Imagine you can try an exact shot squishy would've made in a tournament and, in a real sense, compare yourself to him, given that these trainings can also be shared. I think it'd be very awesome.