Well, that might deal with "buying followers". But my personal pet idea for paying for twitter would be something like reddit gold: a slight premium tier, but not necessarily paid for by the account holder. Encourage people to pay for the accounts of people they like. Heck, provide a "micro" donation system alongside it (although this runs into problems, Twitch passim)

Also, a lot of people would like a better anti-abuse system and some might be prepared to pay for it.