Some other game companies have content in the open world like a cave system or lair that is designed for solo players. Enter one end, work your way up to the “boss” room, loot an elite chest and jump down near or at the entrance. They are short (about 10 minutes), don’t require groups, and are out in the open world (not instanced). They can be built directly into the map (just like the new caves, though longer, were done for the Winter Convergence). They don’t have as much loot as a five-person expedition would have, but its content designed for solo, casual players to have fun with, too. Each region could have three or so such solo-content “delves.”
Perhaps even a weekly, repeatable quest(s) to go with them would be good, too.