I think we will see a similar situation as 3DS vs PSP, meaning Nintendo's next handheld will be moderately more powerful than Vita but with a slightly lower resolution. 854x480 is what I'm expecting.
I also think Nintendo might start doing more mid-gen upgrades like New 3DS. Every 2-2.5 years or so Nintendo might make a slight increase to specs and increase screen resolution but by having the same architecture/operating system, all games will still be playable on each iteration.
Spring 2016-handheld launch, 5 inch/480p screen, 2gb RAM
Holiday 2016-TV version launch, 1080p, 4gb RAM
Summer 2017-handheld XL-6" screen, Mini-4" screen
Holiday 2018-upgrade, 720p screen, 3gb RAM, faster CPU
Holiday 2019-TV version upgrade, 4k resolution, 8gb RAM
Holiday 2020-upgrade, 1080p screen, 4gb RAM, faster CPU
Something like this could potentially happen.
When the herd loses its way, the shepard must kill the bull that leads them astray.
