Bob, I suppose - in a way - that they already do. For instance, a 'missing feature' in a piece of software for which it's obvious the feature should be there can be considered a bug. It's something that should work, but doesn't... not much different than omitting a line of code in a routine. A plugin that transparently adds that feature fixes it. <shrug>
I'll bet that plugins could also supplant existing functions -- actually overwrite their menu presences, so that the plugin repaired a problem in the original function. I've never tried, but if a menu item can be added, then it's reasonable to believe that an existing one can be replaced or have its links changed.
Lloyd