I think there should be at least some multiplayer achievements, such as winning X times, winning X times on an X gamemode, or maybe winning by different win conditions (Score, accuracy or combo) or winning on diferent team modes (Head to head, Tag Coop, Team VS or Tag Team VS)
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Multiplayer achievements [duplicate]
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More achievements the better 
Although I think there should be more of an incentive to unlocking them, such as unlocking a particular super-secret peppy-created map for each achievement unlocked
Although I think there should be more of an incentive to unlocking them, such as unlocking a particular super-secret peppy-created map for each achievement unlocked A token system would be really cool! I imagine it'd be similar to FFR's where you unlock special songs for achieving certain cool or strange scores, such as FCing X song or hitting every other note as a 100 instead of a 300. (There's a token on FFR where you have to get a score of 2 perfects, 4 goods, 6, averages, 8 misses, and 10 boos on the song 'Excite Bike')