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First Post
The advantage of going for a middle-ground approach is that those people who want to comb through books can, those who don't can just use book monsters or the simple templates to customize.
If they can do it that WOULD be awesome!

Which is a mixed blessing. It can certainly add some surprise to an encounter.
However, when every monster is unique and does something unique, all the powers blend together. Unless it's really interesting (or really sucks) it's seldom memorable.
Mystery powers work best when they can be applied to well known monsters, as a nasty add-on. When players know it's something special and aren't ready for it.
And sometimes it's nice to use standard powers. When you know how nasty a spell is, it's much more terrifying. And name spells don't need to be described. There's a lovely narrative ease to just saying "the wizard casts fireball."
See I don't agree here. I'm not a fan of it being special by virtue of it just not using standard rules. (In my experience that just ignites a 10 minute battle with the rules lawyers- Especially when its eating their characters spleen...

What I DON'T want to return to is planning by rules approach... IE "Oh it's a fire creature so it will use fireball so if we apply x defense we'll be ok..." Or long drawn out arguments again with the rules lawyer about how X couldn't do Y...
Creatures built using their own rules in my experience tend to negate a large percentage of these types of issues. So I'm in favor of them.
But I'm also not trying to ruin anyone else's fun...So if they can find a way to do both? Awesome!