Hey cloclo031297,
Nice to hear you want to come back!
Luckily, you don't missed very much. The maximum level got raised to 85, but there is no new questline.
A new Observatory Rotation has been added (Hoof [from Winter Palace], Prophet [from Winter Palace], Maverick [from Secret Paths] and Cosmos [new]).
There is now a new type of gear called "Dragon Relics", which can be obtained with Realgar, Battleground Emblems or by participating in Observatory Normal & Hard mode.
And I think that are all major Changes. If missed something, Im sure someone else here will tell you