Not sure if an option to do all content at all levels is the best way to not negate content. It removes the goal of working towards something when leveling. Sorta like how at launch you could wear endgame gear at rank 1... sorta was seen as a huge negative by the majority because it removed the feeling of progression.

Now a level sync option like in XI where you could level up a say level 40 job when synced to rank 25 or so would be good.

I personally would like to see dungeons having multiple paths in them. Ones with the standard level mobs, a mid level mob path, and a really really hard endgame path. That way say Totorak is valid through out the game but we are not doing the same exact run from rank 20 all the way to 50 of the game.