Square Enix had trademarked three names in Europe that hint towards a localization of the Seiken Densetsu Collection outside of Japan.
In 2017, Square Enix released the Seiken Densetsu Collection for the Nintendo Switch in Japan, which consisted of the three Seiken Densetsu games. The first two games were released outside of Japan as Final Fantasy Adventure and Secret of Mana, while the third game was never officially localized.
Recently, Square Enix has trademarked the names "Final Fantasy Adventure", "Mystic Quest", and "Secret of Mana" in Europe (via Siliconera), which strongly hints towards a Seiken Densetsu Collection Release, as Mystic Quest was the name given to Final Fantasy Adventure in Europe.
The big question is whether Seiken Densetsu 3 will be localized and included as part of the collection? It has been speculated that the need to localize Seiken Densetsu 3 into other languages is what has kept the Seiken Densetsu Collection in Japan, as Square Enix probably didn't want to go to the expense of translating an entire game. Seiken Densetsu 3 has been available unofficially in English for years, as it was one of the first games to ever receive a fan translation.
So, what may have prompted Square Enix to finally localize the Seiken Densetsu Collection? The most likely answer is the success of Octopath Traveler, which was a surprise smash hit for Square Enix and acted as proof that there was a strong thirst for retro JRPGs outside of Japan.
The recent success of the Nintendo Switch console as a whole may also have contributed towards the decision, as the install base for a possible localization of the Seiken Densetsu collection is a lot bigger than it was in 2017.
We know that Square Enix now has an entire division devoted to the Nintendo Switch and an English localization of Seiken Densetsu 3 might be one of their first projects. Nintendo might be giving away a lot of their retro first-party titles as part of the Nintendo Switch Online service, but Square Enix is more likely to re-release their retro games as part of collections, especially as many of their games have modern mobile ports that could easily be moved over to the Nintendo Switch.
Will Square Enix finally give the Mana series fans what they have been asking for? It certainly seems like they are planning something by trademarking those names.