When Blizzard looks at a piece of this game they need to go back to basics.
They need to, while making content, remove any rewards from doing them. They need to then make them so fun that people do it, even if it gives no rewards.
Then after you are at that point, you can start adding rewards.
Today, it seems to me that the rewards are what is supposed to make content "fun"? Look at the insane ilvl gained from the weekly Warfront quest. Look at the insane amout of AP from the weekly Island Expedition cap. Look at the daily world quest cache. Look at the Weekly M+ cache. Most people do the minimal amount of effort on these parts of the game, just to get the reward.
Would people still queue for Warfronts if it gave no rewards?
Would people still do Island Expeditions if it gave no rewards?
Would people still do world quests if it gave no rewards?
Would people still do LFR if it gave no rewards?
Would people quest if it gave no rewards?
I think the answer is fairly strongly no.
Exactly why the answer is no could differ from person to person. To me, I would not do any of those 5 things because they are brainless and unchallenging.
Here are some aspects of the game people WOULD do even if they gave no rewards:
Arenas - People that are already "rank 1" still queue because they love playing it.
Heroic/Mytic Raids - Guilds would still do raids where they got 0 loot, the reason they raid is primarily because it is fun to beat the bosses.
Random battlegrounds - Today you barely get any rewards for doing them, but they still are quite popular.
World PvP - Yes we get the bonus today, but World PvP was done with no reward since Vanilla.
Maybe, once you start looking at your system from this perspective, you could improve the game next expansion. Maybe then you would not add systems like Expedition Island and Warfront, where people never in a million years would enjoy them if they gave no rewards.