A perfect MMO, does not differentiate between solo and group players, but unites them all.
What this means is quite simple.
You can go out in the world and hunt a bear and find the best item in the game, as that bear did swallow it. just like you can meet 19 friends and hunt down a dragon and find the best gear in a treasure box.
When MMO´s were invented, this was the way of doing it and you had no advantage by playing in groups. It was quite the opposite, it had a major downside, as you could be killed and robbed by them.
In Vanilla WOW, we still had glimpses of that left, some recipes like Crusader Strike could be found in the open world. However, as more time passed, WOW turned into a group or die system. So unless you group, certain rewards were not available for you, which is a terrible design choice.
Back then, humans were in general more kind and you had a reputation to lose on your realm, so that group play was not as toxic as it is today.
So if we look at WOW today, then we see a system in place, where you have to hit a button to group up and after you did, you are rewarded more than when you don´t hit this button and remain solo.
Delves have copied that system, by tuning it for groups. Yes I know, you can do them solo, but the balance is for groups, which makes the entire system look stupid and who knows, maybe they come up with a Mythic Delve system soon, that you can only enter in a group like they did with Scenarios.
To come back to your original question “what kind of content people are looking for”, well it´s simple.
Joining a group should not be mandatory to find the best gear in the game, or to craft it, or to reach a pvp rank. Everything that you can achieve in a group, must be available for players that solo.
So if a player decides to specialize in crafting, then he should be able to gather all materials and patterns solo, just like a player who want´s to find the best weapon for his class, should be able to do so solo.
It´s a silly system that joining a raid with 19 people, offers you more rewards, than playing an entire year solo, doing all the quests, discovering all the areas…
The entire system of going out into the world, hunting down animals for leather, looking for herbs, entering caves with a chest is broken and needs to come back like at the start of Vanilla, where exactly this was rewarding content for solo players.
If groups no longer gain better rewards, then this would also help the community, as I do believe that many grumpy and toxic people in groups, don´t even want to be there and only join because they are forced to do so.
This would create a less toxic atmosphere if we could get rid of them as then, only people would play in groups, who want to socialize with others.
I agree with this,
playing in an MMO means, you interact with others. The question is just, on free will or by force and right now, WOW is doing it by force, when all rewards are locked behind the group system.