I'm passing the time by coming up with concepts for baddies. Nothing really concrete, just ideas floating around in my head. One idea that I thought was simultaneously funny and scary in the context of the game is a villain that realizes that things in his universe aren't as they should be. Perhaps he got a glimpse of our world, or another game system, whatever. He just knows that there are rules to his universe that bind everything, and he's tired of living by those rules.
Is it even possible to run a character that's aware of the rules underlying the RPG that's being played? How would such a villain fight? I imagine a lot of metagaming going on in his mind, which could be fun if used correctly or very annoying if abused.
Better idea- a bard that interprets everything in terms of fantasy tropes and cliches. This would allow you to comment, and have an 'outside view', but it would be enough in character to not be terrible.
"oh, the big strong swordsman has come to save his princess from the evil mage. How beautiful. It would be more beautiful if you didn't stop at every temple of calistria along the way here. But I am sure you will beat me and live happily ever after... or until she slips some drow poison into your glass of wine."
Another thought I have that could connect with this, which could help you downplay 'creatures'- don't have the villain be a 'person'. Have them be a "character".
Set up- a cult of evil bards join together, and they all assume the role of a single, well defined "character". They understand his perspective, motivations, and operating procedures, and each and every bard could keep up the game. You could kill any single bard, but the next bard would just pick it up, and act they are the same person. This isn't possession and switching bodies, but copy cats that put in the effort to create a continuity of the "character". Immortality through performance.
overall- you can have a boss, and even let the players kill the boss in round 1, but someone else just steps from behind the curtain and lets you finish the monologue. A useful tool for a GM.
This "character" could have been based off of a 'real' person that was obsessed with performance and predicting the flow of stories. Through manipulation, he has set up an organization that keeps the "character" alive. And each cult member is a master manipulator that can bring many different groups against the party members.
IMO, the GM must strive to be a neutral arbiter of events at all times.About the only reason a GM can deliberately metagame is when they are playing the part of a seriously superior intelligence and even then it must be limited to what the creature would be able to deduce or induce.
I've seen accidental metagaming too often and been guilty of it myself. Pre-rolling was my sin. I intended to speed gameplay but found that I would be tempering my actions based on the rolls coming up. So I stopped pre-rolling.
I don't think that having a villain (or any character) who is aware they're a fictional character will necessarily lead to metagaming, though I agree it is dangerous.
My thought for such a character- they're aware of the fact that they exist just to be killed by the PCs and become nihilistic. Maybe despondent, maybe they start acting in ways that would seem like they're insane.
How different is it really to know that you're a meaningless pawn of unknowable entities whether they're lovecraftian or just some nerds.
Said character wouldn't even have to ever tell the PCs their dark secret.
On the other hand having a character abuse game mechanics and metagame is only on the table if you've all agreed beforehand it's that kind of game.
I agree that springing that kind of thing of the your players in an otherwise normal game is a Pandora's box you won't be able to close.
Anecdote from my last homebrewed campaign where something similar happened :
We found a Deck of many things that the GM had modified to add non existing cards. We never never pulled anything too bad but in one of the last sessions our very naive and simple minded swashbuckler pulled the "You permanently break the 4th wall" homemade card.
His change of personality was so brutal we considered his character dead inside, so it was a pretty bad idea.