I’ve always liked the idea of doing side quests just for fun on my main or to level an alt, but I rarely get intrigued or incentivized enough. So, I leave all those cool quests unplayed. What if you instead combined side quests with archaeology each expansion? So, each expansion has a built-in “deep dive” into the lore even further by literally “digging” into it deeper through archaeology, which we only get access to by doing the side quests that give us reputation with archaeologists and increased levels of gold compared to regular story quests.
The incentive is that the side quests lead to a questline of archaeology quests that allow us to dig up transmogs for armor, weapons, gather up pieces for a mount, get gold, etc. Then, from there, we can have evergreen archaeology digs like the old days where we can farm gold and also have chances to find common, rare, epic, legendary, or artifact level pieces of archaeology themed on the expansion. It makes it feel a lot like real archaeology where it’s gold, lore, treasures, and random finds.
Most old content already has archaeology and story quests, but maybe you could apply the same reasoning from above to old quests as the archaeologists make new discoveries in old content (as shown through side quests in old content) since each new expansion’s lore knowledge unlocks better understanding of the old world for them and new secrets for us to unlock there. It’s already using all built-in technology, quests, etc. but combining them in a focused way that encourages side-quests to be played more, expand the lore, possibly open up the old world, and bring back archaeology as a highly engaging part of the game.