Unless you're going to completely neglect individual rationality, then you're probably going to have to rewrite a lot of the Buffyverse too. (Not just because of out-of-universe caused things, like the 'guns=bad' thing it had going, but also because of the characters routine neglect of in-universe methods of quick and easy victories.)
Though it might be interesting to just write a '24 hours to victory' type scenario.
It would have to be a heavily-AU, from-the-beginning-but-everything's-different kind of fic. I have a pretty good idea of how I'd want to tweak each of the main characters (and villains) to be much, much scarier.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: