I have recently read a lot of blogs on this exact issue. After all the points of view and insight, what I would suggest is show a link to your new feature, but

I have recently read a lot of blogs on this exact issue. After all the points of view and insight, what I would suggest is show a link to your new feature, but make it an active link do not grey it out. The key with the active link, you can measure how many of your users actually click the link and would be interested in your new feature without having actually spending the time/money/effort creating it. However, as you mentioned the draw back is by activating the link to a feature that is not there you will have to post an under construction page or the mm/dd/yyyy the feature will go live.