There is little strain on the "language leaders" (that would be barking up the wrong tree, anyway) as the features you named with one exception are already done

There is little strain on the "language leaders" (that would be barking up the wrong tree, anyway) as the features you named with one exception are already done and have been available for years. • "is item 'in' set" operator: https://metacpan.org/pod/Syntax::Feature::In https://metacpan.org/pod/Syntax::Feature::In • sets: https://metacpan.org/pod/Set::Object https://metacpan.org/pod/Set::Object • clean function parameters declaration system: https://metacpan.org/pod/Kavorka::Manual::Signatures https://metacpan.org/pod/Kavorka::Manual::Signatures • constants that are not functions/objects: https://metacpan.org/pod/Const::Fast https://metacpan.org/pod/Const::Fast • enums that are not functions/objects: ? • structs that are not functions/objects: pack/unpack, for explanation see https://metacpan.org/pod/Convert::Binary::C https://metacpan.org/pod/Convert::Binary::C