Have a bit more I just learned recently about these games from my sources with localization. Some of these names I didn't really know about recently so I wasn't able to specifically point out what returning characters will be in these games exactly (I haven't actually played a Pokemon game since Heart Gold).

Let's GO information
>The new Eevee evolution, Ceegeon, is set to be revealed very soon worldwide in the next few weeks. It looks like a Porygon x Eevee hybrid (don't worry it isn't, lol).. This will be the solo new Pokemon in Let's Go.
>Gladion and Lillie are in the game. Gladion is in Pikachu ver, Lillie is in Eevee ver.
>There's a hub for players online to interact and battle/trade with each other. This is what replaces GTS and random battles. All the features of both will be featured here but it's more social. Voice chat and everything.
>In addition to local co-op, there's co-op missions you can do online with other players where you fight bigger versions of other Pokemon as boss fights
to be continued....