Pokemon has filed a trademark for Tag All Stars in a legal category they’ve use for set names in the past. Over the years the names of many Japanese sets have been discovered through trademark filings, most recently in the cases of GX Ultra Shiny and Miracle Twins.
The trademarks are usually discovered around half a year before the set releases. The filing for Ultra Shiny was first discovered on April 20th, 2018; GX Ultra Shiny was released seven months later on November 2nd, 2018. The trademark for Miracle Twins was discovered on November 27th, 2018; the set will release on May 31st, 2019, or six months after the initial trademark listing.
Therefore, Tag All Stars will likely be Japan’s fall set in October or November and finish off the Sun & Moon series. The first Sword & Shield series set should release in Japan in December and then in America in February. This is based on past release patterns, so we’ll see if it holds true again this generation.
The set name implies it could be another reprint set like The Best of XY, GX Battle Boost, etc. If so, it’ll likely feature a bunch of reprinted Tag Team Pokemon. If they want to go out with a bang, they could feature new artwork or even be shiny versions like in GX Ultra Shiny. We’ll see!