You don't need plugins for that. Take for example git-svn (perhaps not the
best example, as it is Perl script; but Python although has compiled form
is script language at heart), which went AFAIK from external contribution,
to being in contrib/, to being in mainline (and in git-svn package).
About plugins modifying some core functionality: this is rather sign
of not attracting developers to do it in-core...
ShadeHawk on #git