All the way back in 1999 Everquest had mobs that would path through entire zones, you never knew exactly where they were and you'd have to "hunt" them down. EQ also had super rare mobs that would only pop and path maybe once every 2 weeks as well. Finding and killing one of these mobs for a chance it's rare loot could drop felt very rewarding and gave you a purpose to go looking for them.Can you please provide a list of MMOs you have played that have that.
Just about every MMO I've ever played had harvestables that randomly pop up in different locations. Be it Everquest, Everquest2, WoW, ect... they all had nodes that once you picked them they would randomly pop up somewhere else when they respawned. EVE Online (although a different kind of game) also did this with data sites, relic sites, gas clouds, wormholes they all would pop up somewhere different once they despawned. You must have this variety and rareness and randomness for a game to feel exciting.
When things don't roam, when there are no rares and nodes are static, exploration becomes extremely stale very fast once you've been around the map a few times. You gotta mix it up to create that feeling of excitement of not knowing what you might come across to make exploration fun.