First and foremost it needs to look good as a twitter logo. Both on the twitter website as well as in the most relevant twitter applications.
For the past year or so I have been using the logo from the German comic "Werner" by Rötger Feldmann. Rötger has now, rightfully so, asked me to replace it.
I have attached that logo.
I really like that logo, also because I like the werner in the comic very much. More info on that comic: http://en.wikipedia.org/wiki/Werner_(comics)
The new logo does not necessarily need to be like this, but something that just has the W in there would certainly be OK. If it has the rounded, glossy feeling of a button, like the current logo, that would be prefered.
But I am open for more creative solutions as well.