You’re right about the spawning dragon. I was being terse for clarity’s sake, but a more honest open-world example would look like this:

• Dragon is spawned from the start of the game
• Player can find it and kill it at any time
• If the dragon is dead and player visits the mayor, there will be a small token of appreciation (probably smaller than full reward, because “what are you gonna do, unkill the dragon?”)
• The gating (“player must first find out about the lair one way or another”) works on the dragon’s exploits in the region, and therefore the introduction of the reward on its head. Until the quest is interesting, we won’t offer it. Until the quest is interesting, the dragon keeps hidden in its mountain. (Alternately: until the quest is interesting, the dragon doesn’t do enough damage to get the mayor to promise a reward for its head.)

Of course, this also needs more work, more writing, and more code (and so potentially more bugs). It could even be combinatorially more complex (if the mayor giving you the quest and/or the reward is affecting future developments). But, if given the chance, I’d go this way.

Thanks for mentioning locked areas. They’re really useful. I like how Zelda: BOTW uses something similar without the “invincible guard” thing (spoiler alert: you start on a high plateau, so without a hang glider — which is an important part of the game’s mechanics—you can’t come down). The problem with barriers is that you can only use them a small amount of time, and in spacial-related scenarios. Otherwise, the player gets annoyed.