Imagine if, in one timeline, you could have
the people you follow regularly
all as one River of Tweets. The user interface for choosing what shows up in your timeline is done with checkboxes, à la Checkbox News.
So if you're going to a ballgame one day, maybe you'd follow everybody in your regular timeline + your Los Angelino friends and a couple of searches around the California Angels (I still call them that). You'd follow your mentions, but an hour after the game you'd want to turn that off for a couple of hours. Then when you got back to your hometown, you'd switch gears and have your Windows Mobile friends (a list) plus your regular timeline plus mentions. All programmable of course. Weekend mode is just a subset of your timeline (a list) + mentions, no search terms that happens to start on Friday afternoon and switches back to whatever you have scheduled for Monday morning.
http://drupal.org/project/twitter (3.x branch)
Maybe some crazy http://drupal.org/project/transformations action
The client, in my mind, is web-based with mobile skins, powered by Drupal, mostly because that's what I happen to know best. The API would, necessarily, be built first, then have the "front-end" use the APIs. That way, the programmable part is already built!
I also like the "weekend mode" idea. http://inessential.com/2009/12/13/weekend_mode_for_twitter_client which is really "just" an extension built on the programmable Twitter client aspect.
River of News: http://www.reallysimplesyndication.com/riverOfNews
Programmable Twitter Client: http://blog.rsscloud.org/post/260975572/we-need-a-programmable-twitter-client