Virtual goods. Go super edgy, forget flowers.gif and hugs.png: vices sell, introduce liquor, hookers and hard drugs. Maybe even go deep on the cultural references, even be a little harsh and xenophobic. Virtual gifts have a general positive connotation because they're usually given to friends, on facebook and other closed social networks. Twittering is a place for adversaries, a perfect play ground for irony and everything anti.
As a super premium: allow people to upload their own text and graphics for a dollar, and attach that to the tweet.
@squarejaw bought *victim* a pair of nads
@ironical sends you a touching violin solo on a miniature DVD
@citygirl sends you another pair of pink polo shirts to wear over all your others
etc.