Song fic: Like Nansi_Alexander so eloquently said in this awesome badfic entry; "It is also NOT a song fic, even though there are some song lyrics here, but that is not because they are like, SINGING, it is because they are about Best Friends which is what John and Rodney are, only not exactly, because they are totally soul mates only I couldn't find a song about soul mates so I had to use this one, which kind of sucks, only not because I guess they're friends too."
That sums it up quite nicely.
Poetry fic: There are only two characters in my fandoms I can easily buy writing poetry. Spike and Sheppard!
...Sheppard you say? Look it's obvious. He's totally, secretly a fourteen year old goth girl, who can't talk about his feelings, so he writes these great angsty poems in his secret diary, that no one ever gets to read, except maybe Rodney when they're both drunk. Only Rodney totally mocks him for it, so then John has to listen to black sabbath and cut himself... What? That could totally happen!
But really, I'll belief a lot if you sell it right. And If you do insist on having your characters act like beatniks, just tell us they're doing it. There's no need to post the actual poems, because I sure as hell ain't gonna read them (but I will concede I'm biased, cuz well... I just don't care for poetry)
The What if Series rewrites: This trope I don't really despise as such. It just keeps disappointing me in crushing ways. Usually the authors have a really promising plot bunny. Adam Pierson is found to be immortal and send to Sunnydale to help the slayer! Rodney is a fucked up Sentinel oh noes! Sheppard can whistle the national anthem through his nose!
The fun in this trope is to see how things will turn out now, with these radically different characters and backgrounds. And again and again the answer is... exactly the same fucking way. For some reason the authors never seem brave enough to stop slavishly following the episode plots. The worst of these offenders even seem to be copying lines directly from transcripts!
Nothing changes. Their cool revamped characters hardly make a difference, even when they clearly should. And it pisses me off, because the little bits of original fiction that do sneak in there tend to be awesome.
Seriously, If I want to read a what if story where nothing fucking changes, I'll just pick my Wicked Willow series of the shelf.