Definitely an interesting notion. I too have been following Twitter and its Bartimaeus-que ability to shift shapes. Right from being able to add an expense thru to Xpensr, or being able to add a reminder to Sandy..Twitter seems a ubiquitous endpoint, much like a telephone, acting only as a conduit to carry commands to a given destination.
But this openness is not something that can be built consciously. It is difficult to predict which application would be more amenable to adaptation by users in brand new contexts, short of actually rolling out a product and seeing what sticks!
But a simple rule of thumb could be used-
i) Start from simple systems
ii) Change minimally when confronted with new need
iii) Yet retaining backward compatibility
v) Repeat as required
Numerous examples come to mind to illustrate this but HTTP, TCP/IP standout as familiar examples. Anything that aspires to universality, it seems, ought to heed to the principles of evolution.
Bottom line, no amount of upfront planning is going to tell you with any certainity if users are going to ‘play’ with your application. Starting simple, listening to customers and a willingness to drop pre-conceived notions can inform whether the product you have is a framework for serendipity!