First of all, we need to rebrand the application name to something longer. Let's call this Windows Live Gmail, and add some of the visual elements connected with Windows. Also, as in Hotmail, there needs to be less space for the e-mail subjects to make place for a reading pane, which is full of verbose explanatory help text.
Ouch. But, wait, it only gets better:
For another design iteration in our inbox, we will need to camouflage the checkboxes next to the messages by putting a mail icon on top of them. Also, we need to break up messages from conversation threads into their individual parts. Furthermore, this version of Gmail needs to change from context-aware text ads to context-unaware graphic banners, which we'll require to carry at least one clip art. Gmail currently has a chat box which I don't use and thus find annoying, so I think we can build on that and expand it to a more full-featured chat widget, replacing the labels box. We'll also adjust the spam filter slightly to show a couple of more bulk mails in the inbox.