Well that was quick! Yesterday there were new registered trademarks and information has already leaked out for the new upcoming summer sets.
Update 28-02-2020: Added digital images + Eternatus Gigantamax Deck sleeves + Deckbox dummy image.
Update 7-04-2020: The set name S3 World Down has been changed to Infinity Zone (literal translation is Mugen Zone, Mugendina is the Japanese name of Eternatus) on Amazon JP. No reason or confirmation given from Pokémon Company.
S3 World Down was also trademarked a while ago and will be the next main set and will feature Eternatus V and Eternatus VMAX. It seems this will be Darkness type. It's also been told that this set will focus on Darkness type and will include cards that will support Eternatus VMAX.
This set will feature 100 cards + Secret Rares and will release on June 5, 2020.
Next is S3a Legendary Heartbeat, the set after S3 World Down. This set has Celebi on the dummy pack art and will contain 76 cards + secret rares and will release on July 10, 2020. 20 Packs total. Each Pack includes 7 cards.
This set will most likely be related to the upcoming movie Pocket Monsters the Movie: Coco. It will release on the same day as the movie does.
This set will contain Amazing Rare cards. Amazing Rares are powerful Pokémon, but you only take 1 Prize Card if you defeat them.
Along with this, new V Starter Decks will also release on the same day as the S3a Legendary Heartbeat's release. These will cost 500 yen and will contain a V card and "powerful" Sun& Moon trainer cards.
In 2018 they released similar GX Starter Decks that would cost you the same price as the V Starter Decks 500 yen ($4,50). The special thing about these products is that they feature 1 guaranteed GX, but they also would gave you many playable trainer cards along with them, all in a 60 card deck.
These decks were massively popular in Japan in summer 2018 and they have confirmed that this product has contributed to many new players and sales for their merchandise and other Pokemon Card Game products.
They want to recreate that success with this product and bring in new players like they did in 2018. They will heavily promote this product according to this image.