The Pokémon Company held a major press conference in Tokyo this evening with a lot of stuff coming in 2019 and some of it in 2020.
Pokemon Go Plus Archives
Amongst a larger description of how the mobile device game will work, came a bit about an add-on called Pokemon GO Plus.
Here's what it is:
"A portable device called the Pokémon GO Plus will enable Pokémon GO players to enjoy the game even while they're not looking at their smartphones[...]