The folks on ResetEra claimed that it is tied in to a certain accessory aimed at children and/or those that do not have a lot of experience with gaming consoles.
Here is what my theory is (more like wild musings and whatever this reveal is will most likely not turn out to be anything like what I am going to say here).
They are going to make use of the Switch's modularity to build some sort of robot-like device (potentially a DIY robot or a line of robots for kids) . The Switch console could connect to the robot accessory wirelessly or through the rail system. I predict this because of the passive walking device/passive walking module (module, implies it ties into a core device that is modular, much like the Switch) that Nintendo patented in December 2017. Kimishima has also said that he wants to use the IR camera on Joy-Con R for something this year; now obviously he could be talking about a game experience but he could also be talking about a robot like device with Joy-Con R used to detect depth and surrounding objects (the Joy-Con could serve as an arm). The user would be able to interact with the robot with Joy-Con L held side ways like an NES controller, potentially voice (if it has the AI for voice in its software and a microphone in the Robot's body), real objects through the IR camera, and Amiibos (the NFC reader is also placed in the hand like Joy-Con R).
Now the more obvious (and less exciting, for me anyway) reveal could be a slightly different Joy-Con with fewer buttons and more emphasis on motion or maybe some sort of camera joy-con. But none of these really fits the kids audience, at least in my eye, they are going after (the simplified motion controls for motion games, etc. seems to fit more with the much older Wii audience, like middle age and senior people). Then again, this is the cheapest route and the one Nintendo will most likely take (imagine how much a robot body would cost).