What ho,
Controversial? Maybe.
But wouldn't it be rather cool if there was a solo-mode available for every new piece of instanced content? Scenarios, dungeons, and even raids.
Rather than out-levelling content before approaching it for mounts, transmog, and so on - we'd take it by storm when its new and challenging.
Rewards would obviously have to be tweaked to ensure that endgame loot primarily came from group-based raiding, but this would still be an excellent way to challenge players and keep us busy while not raiding. Loot could come in the form of reputation bonuses, Valor, as well as the ordinary mounts and transmog items you'd otherwise expect to find in there. But the primary reward would be the sensation of having beaten the content better than last time, and better than everybody else. It'd be the true challenge mode, and for once the solo player would be the hero of his or her own adventure.
I stress: group-based raiding would still be the main allure for challenging endgame content and big rewards. But it doesn't really make sense that only a select few raiders with 16 hours / week on their hands should be able to find the wicked lore-items and mounts from the final boss. Or that only those inclined to join a serious raiding guild should be able to claim the glory from beating the content at the highest difficulty-level, while solo-oriented players are stuck with not having any sense of achievement from beating it through LFR.
While I do want to beat the content with a raid, and have that be my primary endgame - I definitely wouldn't mind first beating it all alone and claim the story and achievement as mine. I think we all see ourselves as the hero of the plot, but strangely enough we're hardly ever alone when it really matters. I'd also love to be able to measure my progress across the expansion by the difficulty-increases I can make while soloing content. It would make the gear-treadmill less pointless, and put more agency into my hands.