TL;DR 
Is like a multiplayer, but instead of other people, your replays.
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So imagine you want to beat a highscore, but you have to continuously check it so you can see what is the difference of score/precision in some parts of the songs (like checkpoints), and when trying to beat it, you have to check your score instead of just seeing a nice flash (like in multiplayer) when you beat it.
But, what will be the difference then?
For now, you can only see when you have completly beat you're score... and that's good, but maybe you did bad in some parts and in others you did extraordinary, so that's why you beat it. With this "request" you can always see when you're doing best, right beside the score, giving you more information on your improvement.
Also, you may be able to download/copy replays from other persons (like highscore ones or from you're other friend), and compete with them.
Extra details
MOD: It would be nice to have a mod that gives you a game over when you have X, difference with a replay.
MULTI-REPLAYS: In multiplayer you just don't compete with one person (most of the times), so having lots of replays could also be cool (There may be some lag though)
Is like a multiplayer, but instead of other people, your replays.
--------------------------------------------------------------------------------------------------------------------------------------------------------
So imagine you want to beat a highscore, but you have to continuously check it so you can see what is the difference of score/precision in some parts of the songs (like checkpoints), and when trying to beat it, you have to check your score instead of just seeing a nice flash (like in multiplayer) when you beat it.
But, what will be the difference then?
For now, you can only see when you have completly beat you're score... and that's good, but maybe you did bad in some parts and in others you did extraordinary, so that's why you beat it. With this "request" you can always see when you're doing best, right beside the score, giving you more information on your improvement.
Also, you may be able to download/copy replays from other persons (like highscore ones or from you're other friend), and compete with them.
Extra details
MOD: It would be nice to have a mod that gives you a game over when you have X, difference with a replay.
MULTI-REPLAYS: In multiplayer you just don't compete with one person (most of the times), so having lots of replays could also be cool (There may be some lag though)
, didn't found this: 
, i already posted on the ghost thread