WTF no way in hell it should last more then 3-4 month EVER. What exactly do you mean there is so much more to do? There is a new map, new items and the cube. The cube isnt adding that much to grind for. IN addition to that we no longer need to grind for rift keys and there is only so many new items everyone will want to cube.
If your an achievemnt hunter and you cant get everything you want in 4 months idk what to tell you, play more?
The only reason I could see them making seasons last longer is so that they don't have to push content out as quickly. Which is entirely something blizzard would do.