My first fandom was LotR. I don't focus so much on ships as I do on characters, and I read mostly fics involving Boromir, Faramir, Aragorn, Eowyn, Arwen, Elladan and Elrohir, (not as a couple though), or miscellaneous elves. I used to read Legolas, but the fangirls pretty much killed that one dead.

I also got into:
The Mummy (never written anything) where I read pretty much anything that didn't involve Rick. Evey/Imhotep, Evey/Ardeth Bey, and some that involved OFCs. Not the posh-est fandom, but...
Troy (never seen the movie though): my ships are Hector/Paris or Hector/Paris/Helen.
Labyrinth: Sarah/Jareth (not that there's much else you can get out of it. Anyone who has written Sarah/Hoggle deserves to be voted out of the gene pool now.)
Star Wars: Padme/Obi Wan
X-Men: Wolverine/Rogue, though I've read some Scott/Jean.

How long will HP last? I don't know...I can only hope that it stays interesting a good while longer. As for flash in the pans, I can't say for certain. I don't pay much attention, if any, to the fandoms other than those listed above. I've dipped a toe into Peter Pan (Hook/Wendy) and PotC, but I don't think I've ever read a whole full-length fic in either fandom. I'd actually like to read some good PotC, but ff.net is chin-deep in craptastic fic, and I gave up.