Something I've been thinking about for a while now is how utterly 'mundane' almost all legendary quests are. What I mean is that the quest itself isn't particularly engaging or challenging or fun. Sure you have those cool little solo challenges/scenarios once or twice, but probably 95% of any legendary questline is just collecting orange 'fragments' from raid bosses that you'd be killing regardless - you aren't required to do anything you wouldn't already be doing and this is what makes them feel boring to me.
It feels like legendary quests are stuck in a rut, and I'd like to suggest some things to really get the players more engaged in these quests and try to make them a bit more fun and compelling to play.
Any legendary quest is traditionally comprised of around three sections or phases, and typically they are all raidboss collection quests but I'd like to replace one or more of those sections with special world content and/or special scenarios. So for example one section of the questline will incorporate elements of patch 5.1's weekly lore quests, followed by a special outdoor solo boss and event.
Instead of raidboss fragment collection, you will embark on a series of challenging weekly quests that will send you out into the depths of a different zone each time where you'll have to locate the exact destination from clues, requiring the player to explore some. Once located you will take part in a phased outdoor event which could be anything from protecting NPCs, fighting waves of mobs or even solving a physical puzzle. After this you will then defeat a special solo boss of reasonably challenging difficulty.
Each weekly quest will engage the player in lore-based events, much like the 5.1 quests. This will continue for 4-5 weeks or so, until you've competed all, after which you will move onto more traditional raid quests.
Another idea is to introduce a special kind of weekly scenario in which you will progress one section of a legendary quest. Similar to the 5.2 Treasure Troves scenario, you will only have one attempt per week, and the amount of quest progress you attain will be based on how well you perform. This scenario, like the suggested world events above, will be difficult, but not extremely hard.
The actual scenario will include elite monsters, obstacles, a bit of platforming and possibly even a puzzle or two. The scenario will be timed but will not have a time limit -- Completing the scenario will always award some fragments/progress, no matter how badly you performed or how long you took. But reaching the end within a certain time will award a greater amount of fragments/progress.
At the end of the scenario, after the timed section, you will fight an extremely difficult boss which you will only have one attempt to beat per week. If you beat him you will attain a greater amount of fragments/progress, but if not you will leave the scenario with however much fragments/progress you had otherwise earned. Most players should be able to complete the timed section, but only some will be able to beat the boss.
The purpose of these suggestions are not to make legendary questlines more 'difficult' but to make them more fun and engaging, to spice them up a bit, so we aren't just doing the same old thing we'd be doing anyway. Obviously the quests should include raid content too, I just don't think they should be 100% just farming raid bosses, as they have been in the past.
I hope maybe Blizzard will see these and be inspired to improve the boring old legendary model.
DISCLAIMER: On the off-chance that Blizzard do notice this and would like to use it I relinquish all 'rights' to this idea and give Blizzard full permission to use it any way they choose without obligation to compensate me.