Not only HD screen, but I think Nintendo will ditch resistive for capacitive.
Nintendo has stated that they feel its "boring" to do the same thing over and over, one of the reasons they cut themselves out of the MSony competition. The DS era lasted 2 gens and its time to shake things up. Nintendo is more likely to return to one screen and implement capacitive, than keep the dual/resistive screen combo.
Lets not forget manufacturing costs. The market's flooded with hd capactive screens, so its easier for Ninty to profit by going with cheaper off the shelf parts, than to create some custom sub HD screen. Nintendo is a cost driven company and even though lower res seems cheaper, that doesn't apply now b/c today's lower end has made significant leaps. In 2016 or '17, I highly doubt Nintendo will opt for anything sub 720p. qHD (Vita res.) is the worst case possible IMO.
As far as a darkhorse candidate, one large, foldable screen could very well be in Ninty's plans. http://www.inquisitr.com/1626745/folding-screens-samsung-said-to-release-revolutionary-bendable-phone-later-this-year/
With phablets growing more popular and Nintendo's rumored fusion, I can see Nintendo opting for a larger screen with the desire of maintaining portability.
