These are actually two quite different questions, because one thing is to be desirable
as a feature to be included into whatever, and one thing is to be likely
. As far as Perl is concerned, in particular, of course Perl5 sticks to backwards compatibility so anything more than negligibly disruptive of it is to be discarded.
OTOH some misc features can be perfectly compatible. For example the dor patch is already available for stable perl and included in bleedperl.
Personally, one thing that I found amazing in Perl6 is its solution of the dichotomy between lexical and package variables by means of a magic package. Now, I think that by suitably choosing a similar magic package, this idea may be adapted to Perl5 without badly disrupting backwards compatibility.
So what is your candidate for a Perl6(-like) feature that you think it would be possible to incorporate into some next major Perl5 release? Of course hypothetically speaking, i.e. ignoring technical difficulties e.g. wrt actual implementation...