I'd like to see the Tweet button shorten the URL through bit.ly versus the standard t.co shortener so that I have better tracking. This would require using the bit.ly API and I'm guessing you would have to have a field for the individuals API key.
I actually like this idea fyi; I'd like to see an "API Keys" section of the CP eventually (to keep things tidy) so we can work on expanding things with plugins which would need api keys.