The Legion invasions have shown us that Timewalking is about to go mainstream. Blizzard has mentioned that there will be no questing from zone a to zone b path, instead you'll be able to go in any order you wish. What I think they've failed to explain is that it doesn't necessarily mean at level 100+.
I forsee content having no borders. You approach a bad guy, he will automatically match your level. This means you could theoretically raid, quest and run heroic dungeons if your gear is appropriate. Or would you need to complete the pre-quests?
Of course, some content will still be gated. You can't have people flying around before level forty, can you?? Oh! Disregard my chauffeur please, and this pair of wings, and the feather from Draenor.
So my question is, why am i bothering to level my bank toon? Next week, she'll be running Siege LFR!! Leveling via all possible dungeons! And maybe even questing in Panda land even though she's not even level 30 yet.