I've got another few Idea's:
a)Make a cumulative total of 5 or more Maps That have been Ranked.
b)Make a consecutive total of 10,000,000 Points in 3 or less Beatmaps
c)Offline Scorer; make a cumulative total of say, 30,000 Points in local mode. Online Scores not counted
c2)Online Scorer, opposite of above. Probably need more online points to really make it an achievement though
d)Level Specific Achievements. Say a Level 1 can only complete Achievements in the top row/left column, and new achievements are unlockable as you gain levels.
(Another Idea; Give low Level players achievements like get all beats of a specific colour in a beatmap. The colour could be generated randomly upon level entry/reset; throwing a little difficulty into the mix. Higher levelled players could get say All Beats on a map.)
e)Achievements could give a point bonus of varying size, depending on difficulty of said achievement
f)An achievement for reaching level 99!
g)More Beatmap Pack Achievements; There's a few there already; why not make more?
h)Get an SS three times in a row (I've even got a name: Hat Trick!)
i)An achievement for posting in relative threads; say, 200 in Feature Request, 300 in Pending maps. Give People a reason to check out other people's maps; then the flood should soon subside.
j)An achievement for completing a beatmap with HardRock, Flashlight and Hidden. Double speed shouldn't be necessary
k)(Ripped from AudioSurf) Beat a friend or foes high score on a song, say 30 minutes after it's been posted.
l)Make a total of 30 friends (Possibly)
m)Play Osu! for say 8 hours straight (the timer's there already, so It couldn't be hard to set up) (Then again, people would just leave their computer on and do somehing else... maybe another timer counting inactivity? when the mouse isn't moving, even fractionally, the timer counts up, and if it is say, 30 mins of inactivity at the end of eight hours, the achievement isn't unlocked)
EDIT: n) Could Have Achievements based on artists. For example, say a person plays 10 Songs played by Band 1. As it turns out, their songs all happened to be on the same album and thus counted for an achievement. I'm not saying per album is necessary. In most cases it's impossible anyways ATM.
o)Could Set up a player ratings system where helpful answers in a forum allow the thread starter to apply points. Sort of like that Yahoo! answers site thing. When a person gets a high enough score, they get the achievement
p)As I think I said in my above post, what about fire Osu! tournaments? Thee could be groups of two per team which play for the prize... a a lolly and an achievement which only THEY can then get. That's a pretty high reward if there is any chance of a more in depth achievements system.
(I'll have to see If I can get to z):P)