I don't think Microsoft would do one. After money bleeding mess that the 360 was lumped in with Windows Phone, Skype, and Bing plus wanting to strengthen Windows as a whole again, they wouldn't want to throw their hat in the portable arena. Plus their mobile direction has already been shown little love. I doubt they'd say "No one's really feeling Windows Phone, we just got Nokia's stuff, we need to bring back N-Gage. The world needs a Microsoft handheld."
As someone has said here before, Microsoft (and Sony for that matter) have a lot of stuff going on and can't really afford to stretch themselves even thinner. While Nintendo catches slack, it's the only one of the three that has that privilege of being a purely game company. If something falters, it's not a call to bail and "Go Sega". If Xbox One or the "Xbox N-Gage" bled money like 360 did for a period of its run Microsoft won't hesitate to discontinue the brand or lessen their presence in the market. A handheld would put more strain on the brand as a whole to succeed.