Would you foresee the "pro" level to add in controls to minimize the "abuse" issue that Wall Street often points at?
At the same time, if the APIs are fully open, third party clients would be able to build more powerful filtering features. In effect that would allow anyone to build entire sub communities on top of Twitter. This is starting to sound more and more like app.net...
I imagine invite only groups, time-limited groups for concerts/conferences/sporting events, location-locked groups that you dynamically join and leave as you move around.
I may just be describing iMessage or Slack at this point.