Pokemon SM is the only game where you can use the newest and the previous mechanics(Z cristals and Megas) and this game takes place after XY
Dexio and Sina show up giving importance to Zygarde(except in USUM)
These games have a kind of connection, maybe they will have it again in Legends ZA
Also, the letter A in the title can be related to the Aether Foundation
I am sure that Alola will have importance in the new game cuz Pokemon BW didnt receive a remake because BW2 already existed, this might happen again with SM because of USUM