And there's a talking dog (though assuredly no grotty teenagers) in my planned Module "The Black Book"... Yes, there's a book in it too, one that makes the Necronomicon a nice, safe, light, bedtime read... Actually there are quite a lot of books, since half of it takes place in the most bizarre library in the cosmos... and the other half...? That would be telling.
Referring to the mention of a "Plants" theme as an example earlier, I fail to see how anything that vague could be considered a restriction... Module set in jungle where the plants turn out to be sentient, Module where strange plants are evolving into mobile, homicidal creatures, Module involving a flower sacred to some tribe who are none too pleased at anyone tampering with it (Black Orchids or otherwise), a Module with considerable enviromental themes, a Module involving a privet hedge maze or a rose garden... I could go on and on and still miss lots of things...
Armed with an imagination, I don't see anything as being a real restriction just so long as we don't end up with "themes" like "Create a Module involving a mad monk walking through a desert calling cacti rude names before being attacked by bandits, jumping through a hole in time, being savaged by an alligator in a sewer before making an unlikely return to his monastery in time for evensong".
I can't imagine us all producing clone Modules just because someone throws a word like "Plants", "Death", "Farms", "Ocean", "Castle", "Dragons", "Hay Bales", etc, at us. Any single word concept can be warped, twisted and generally hit with bricks. I don't think we should get too hung up about this aspect...
Starts planning new Module, "Vengeance of the Killer Hay Bales of Greyfell Moor"...