Some screenshots showing what the official Twitter client looks like in Windows 10 have appeared in the app's Windows Store listing. The app itself is not available for download, but the screenshot gallery is clearly visible.
The screenshots themselves are very low resolution images. but they still show a new user inteface that seems to be more photo-friendly than previous versions of the Twitter app made for Windows 8.1. Hopefully we will learn more about this app when it gets officially released to Windows 10 users. At the moment, it's not clear when that might happen.