I think it would be great if Blizzard made it possible to join a scenario alone without anyone else.
They could reward you with raid-itemlevel-loot if you can solo the whole scenario. It should feel just like the solo parts from the rogue-legendary-quest. Maybe even a little bit more difficult. I would LOVE to test all my new utility spells during an actual challenge.
They said you won't need a healer to clear a scenarios => it should be soloable if you know how to play your class.
Plus I read a bluepost stating that they want to implement some sort of challenging solo content.