Paul, My vote will surprise no one.
in my opinion, it would be nice to have
1) an eventually huge XSLT file. It's probably easier for most people to comment out or tweak things that are already there than to add the "extra" fields they need from scratch.
+1 The other option would quickly make it impossible for people who are concerned about MARC display (people like me, for example) to contribute what you call "bugfixes". I use fields most other libraries do not, so my stylesheets will diverge. Also, I would argue that not displaying as called for in bib standards *is* a bug.
2) a "local" XSLT file that survives updates unharmed where people can put the files they tweaked to their likings (I think this was talked about while ago, I don't know if it exists already).
This would be nice, but at present it is not possible, and I don't think anyone is working on it. Regards, Jared