Let’s GO Johto has been confirmed but not announced and here are the coming twists.

It’s not Togepi and Marill or
Let’s GO Gold and Silver.

Titles are Gen 8 Baby Pokemon.

It's not a DLC! GAME FREAK wants games!

> Although DLC is planned for Let's GO Pikachu/Eevee new costumes and bike

> Let's GO Johto will be brand-new games set in the Johto Region!

> It will feature Pre-Evolution Pokemon from Gen 8 like Meltan was out of place dex wise.

> Pokemon Let’s GO Luga and Hoiu (concept pre-Evolutions of Lugia and Ho-Oh) as mascots.

Innovation meets Tradition even for Let’s GO.

Source:

http://nintendoswitchdx.com/pokemon-lets-go-to-johto-dlc