I disagree. There are many cases where a default constructor taking zero arguments would be pointless.

I think a better approach would be to allow on-the-fly topicalization of the class in the declaration so you could say something close to

my Dolphin $friend is topic = .new("Flipper");
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Admittedly verbose, but there's no duplication.

I've no doubt the real Perl6 heads would come up with a different solution yet, but I believe something along these lines would work better than just plain autovivification.

Makeshifts last the longest.