Could someone explain story of Cantha in GW1 ?

There are some possiblities that Canta is gonna be in new expansion

I would disagree with the idea that Cantha is going to be a new expansion anytime soon, for multiple reasons (plot direction, world map, narrative motivation, etc.). Maybe Expansion 4 at the very earliest depending on the route of Season 5, but I wouldn't bet on it being part of GW2 prior to Expansion 5 (which would be, what, 2021? 2020 at the earliest?). But anyways:

• History of Cantha, pre-Factions
• The plot of Factions
• The plot of Winds of Change, a post-Factions lead in to GW2's state of Cantha (its isolationism)
• Segment on Cantha from Movement of the World

This will tell you the vast majority of Cantha's lore.