Is there a way after you complete the main story content in all the expansions to just change your difficulty and just play on for harded fights and better loot without having to repeat the story over again?
Yes. Once you have unlocked Ultimate, go to Ultimate, into the Forgotten Gods DLC.
Downstairs near the blacksmith, there is a vendor who sells consumables (Merits) that unlock Ultimate for a character (and give all the important quest rewards like inventory space and skill points from the previous difficulties).
If you use that on a new character, you can start a game in Ultimate from lvl 1.
Well, merits let you skip around (unlocks all riftgates) quite quickly during the story, so you can fight whoever you like for the challenge. For Ultimate difficulty, however, you must complete the story the old fashioned way, walking.
Ultimate SR will be quite a challenge; there’s always super bosses to tackle as well. Or Crucible game mode if you like wave based, timed approach.
OK, I would love it if the game had an adventure mode for completing all difficulties. Letting you take your level 100 character on a free roam in all difficulties.
Well, you can already play the game over and over, farming bosses and whatnot on every difficulty, even after completion. So I’m not sure what you’re suggesting is any different than what we have?
The new expansion (Fangs of Asterkarn) will add something much like that 
It’ll give basically NewGame+, and you can already do Elite at L100 if you want to but it’ll be easy.
Okay, where can I read about this in the notes, please?