I think that world content (world quests, daily quests, callings, invasions, etc.) and queued content such as daily random Heroic dungeons could be a source of currency that could be used to purchase or upgrade Normal or Heroic iLvl gear over time, sort of like the old Valor system. (The current new Valor only affects M+.) If the item level is decent, it’s OK if the currency cannot be used to purchase tier pieces.
For players that like to queue for unrated PvP, Honor gear could be made more competitive in iLvl, or Conquest gear could be made competitive in iLvl without the need for ranking to upgrade.
Expansion features such as 3-man scenarios (MoP), island expeditions, Horrific Visions, or Torghast could be made queueable or scale to solo players, and either drop gear or the currency for gear.
There are many things that casual and solo players are happy to do, but gear needs to be an intrinsic reward. Most solo and casual players will not demand a full set of max-iLvl BiS gear, but what we seem to be getting in 9.2 is way too much of an insult.
World content and daily Heroic dungeons would probably never be too difficult. If world content requires a group, game devs should put an automatic group-finding function (green eye?) on it, as world boss world quests have now. (Ve’nari weekly quests in the Maw in 9.0 for example did require a group, but had no automatic group finder.)
I would imagine that the difficulty level of expansion feature instanced content should be somewhere around Torghast Layer 9 in 9.0, or Horrific Visions in 8.3. Solo players should learn how to CC, interrupt, watch their health, and juggle damage abilities effectively, but if the content is too tightly tuned (too much mob damage or enemies abilities that need to be interrupted come too frequently), then the content could become frustrating for too many players or certain classes.
With world content and queued content, all solo players would have to remain at the same gear level, unless you add in upgrades for players who keep playing to earn more currency.
With something like Torghast, island expeditions, or Horrific Visions, higher difficulties could award slightly better gear or more currency for gear. Players that want more challenge or better gear can go for it, or players that don’t can stay on the lower difficulties.
But the base system should never be as difficult as timewalking Mage Tower, unless it is some kind of additional challenge mode where the reward is guaranteed Heroic or Mythic iLvl gear.
If there are a variety of difficulties with rewards increasing in iLvl, players will be able to earn meaningful rewards in line with the level of challenge they are comfortable with. Most importantly, players will not feel like they are being excluded from progression entirely because they don’t have a team.