All of them, actually.

No Perl 5 framework I know of uses any Perl 5 features that are clumsy in Perl 6.

For example the Catalyst approach of using subroutine attributes for dispatching can be replaced with subroutine traits, so for example you could write (mimicking an example from Catalyst::Manual::Tutorial::03_MoreCatalystBasics):

method list($c) is Local  {
   $c.stash(books => $c.model('DB::Book).all);
   $c.stash(template => 'books/list.tt2');
}
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In fact subroutine traits work in Rakudo today, so the only thing that prevents people from porting Catalyst to Perl 6 is speed and development time.

I don't think other web frameworks heavily rely on intricate Perl 5 features that aren't yet in Rakudo, so there are many other possible targets for porting too.

Update: As a proof of concept I've started porting CGI::Application to Perl 6. Of course it's far from being finished, but I got basic dispatch, hooks and error mode running in with half a day's effort (and many distractions inbetween).