In late 2016/ early 2017 nintendo announced the WiiU replacement, it's called Switch.
They claim it's at its core a home console and that it's not a replacement for the 3DS. Two facts seem to support this notion:
A) The software (Breath of the wild and MK8 are homeconsole, WiiU-like games, not 3DS-like games)
B) Nintendo's announced and confirmed support for 3DS in 2017 in form of hardware revisions and some games. Metroid, Pikmin and Pokemon special editions are happening, hell we may get DQXI in the west still. We are also getting Atlus late in the year.
so this year, no, switch does not replace the 3DS, not in any shape or form.
now, looking into 2018 and beyond, the 3DS will inevitably decline, regardless of whether the Switch exists and/or a dedicated 3DS successor is planned or in the works. So 2018 is, for all intents and purposes, 3DS's last last breath. Market saturation and incredibly fucking ancient technology will prevent nintendo from keeping it alive any further.
so the big question remains.
Will Nintendo release a non-switch branded piece of hardware, say, Q4 2018 and/or in 2019? hardware with exclusive games to it, no overlap with Switch hardware?
my bet is No, they will not release, new, out-of-the-blue hardware. my bet is that they will have several switch forms, one probably resembling more of a handheld (clamshell, AC adapter, better battery life, etc)
that being said, anything can happen and Nintendo is probably be taking the "wait and see" approach. they could for all I care, release a mobilelike-but-with-Dpad-and-buttons digital-only device with all of nintendos games from NES till 3DS for sale on the Eshop that would effectively take the role of "nintendo's new handheld" (not saying it WILL happen, but it could, nintendo is very unpredictable)