I produce some very complicated websites using WordPress and I do some heavy customisation with regards to functionality and admin GUI. I try to keep the WP version up-to-date on all of the sites I run, but due to the customisations, although I try to avoid them, a number of core hacks do come into play. When I upgrade a site, it's a very long job to double check every change and make modifications according to my log and the WP changelog.
What I would really like to see is a way to override core files, built into the core. For example...
could be overridden by
Then I can just upgrade my hacks to work with an upgraded core and not touch the actual core. It just seems tidier and a clean way to do it.
I know this sort of thing is probably not a good idea and possibly open to abuse, but it's an idea all the same :)